MECHANISM 

'•  OF  ';     ' 

STEAM   ENGINES 


BY 

WALTER    H.   JAMES,    S.  B. 

» • 

ASSISTANT   PROFESSOR  IN  THE   DEPARTMENT   OF   MECHANICAL  ENGINEERING 
MASSACHUSETTS    INSTITUTE   OF  TECHNOLOGY 

AND 

MYRON    W.  DOLE,  S.  B. 

INSTRUCTOR   IN   MECHANICAL  ENGINEERING 
MASSACHUSETTS   INSTITUTE   OF   TECHNOLOGY 


FIRST    EDITION' 
FIRST    THOUSAND 


NEW  YORK 

JOHN    WILEY    &    SONS,  INC. 

LONDON:    CHAPMAN    &    HALL,    LIMITED 

1914 


COPYRIGHT,  1914, 

BY 

WALTER  H.  JAMES  AND  MYRON  W.  DOLE 


Stanbopc  ipress 

F.    H.GILSON   COMPANY 
BOSTON,  U.S.A. 


PREFACE 

9 

THIS  book  is  intended  as  an  elementary  treatise  on  the  kinematics 
of  reciprocating  steam  engines  and  steam  turbines.  Sufficient  atten- 
tion is  given  to  the  behavior  of  the  steam  itself  to  enable  the  student  to 
study  intelligently  the  machine  for  which  the  steam  is  the  source  of 
power.  The  indicator  card,  or  pressure-volume  diagram,  is  employed  in 
this  connection.  No  consideration  is  given  to  the  underlying  heat  theory 
or  to  the  details  of  construction  of  the  various  parts  of  the  machines. 

The  book  has  been  planned  primarily  to  meet  the  needs  of  students 
who  take  up  this  subject  as  a  part  of,  or  immediately  following,  their 
course  in  the  elements  of  mechanism,  before  they  study  the  theory  and 
practice  of  heat  engineering  or  machine  design. 

The  purpose  of  the  authors  has  been  to  present  the  subject  in  such  a 
way  as  to  make  clear  to  the  beginner  the  mechanical  principles  on  which 
the  steam  engine  operates,  with  special  reference  to  the  valve  gear  and 
governing  devices,  and  the  various  diagrams  used  for  studying  the 
same.  Examples  are  given  of  the  different  types  of  mechanisms,  these 
examples  being  chosen  merely  to  illustrate  principles  and  methods, 
without  particular  reference  to  their  relative  importance. 

In  dealing  with  a  subject  which  has  been  so  thoroughly  developed 
as  has  the  steam  engine,  it  would  be  useless  to  claim  that  any  new  prin- 
ciples are  set  forth  in  an  elementary  textbook  such  as  the  present  one. 
The  aim  is  to  treat  the  subject  in  a  logical  manner,  as  concisely  as  pos- 
sible, yet  with  sufficiently  detailed  explanations  to  make  the  principles 
easily  understood. 

Chapter  X  describes  the  principle  of  action  of  steam  turbines  in  gen- 
eral and  explains  briefly  the  various  types  of  turbines,  giving  an  exam- 
ple of  each. 

Chapter  XI  treats  of  the  method  of  controlling  the  steam  supply  to 
turbines  and  describes  two  mechanisms  which  are  used  for  this  purpose. 

Thanks  are  due  to  the  various  builders  of  engines  and  turbines  for  their 
ready  response  to  requests  for  information,  for  the  loan  of  cuts,  and  for 
permission  to  make  free  use  of  the  material  contained  in  their  publica- 
tions. Acknowledgment  is  also  made  of  the  assistance  rendered  by 
the  authors'  associates  at  the  Massachusetts  Institute  of  Technology. 

W.  H.  J. 

M.  W.  D. 

BOSTON,  MASS.    October,  1914. 


I 
CONTEXTS 

iNTRODUCnOX T» 

CHAPTER  I 
fl»mw  DISCUSSION  OF  A  RECIPROCATING  STEAM  ENGINE i 

CHAPTER  H 
SINGLE-VALVE  ENGINES.  .  « 


CHAPTER  ITT 
VALVE  DIAGRAMS 39 

CHAPTER  IV 
TYPICAL  PROBLEMS  ON  THE  SODE-VALVE  ENGINE 48 

CHAPTER  V 
GOVERNING  DEVICES  FOR  SINGLE-VALVE  ENGINES 63 

CHAPTER  VI 
RIDING  CUT-OFF  VALVES  AND  THEIR  GOVERNING  DEVICES 7S 

CHAPTER  VH 
MULTIPLE-VALVE  ENGINES.  ,  95 


CHAPTER  \~U1 
HAND-OPERATED  REVERSING  AND  CONTROLLING  GEARS in 

CHAPTER  IX 
VALVE  SETTING iaS 

CHAPTER  X 
STEAM  TURBINES ijS 

CHAPTER  XI 
TURBINE  VALVE  MECHANISMS  AND  GOVERNORS 156 


INTRODUCTION 


THE  steam  engine  is  a  machine  by  means  of  which  steam  is  enabled 
to  do  mechanical  work.  The  steam  ends  of  direct-acting  steam  pumps, 
steam  drills,  steam  hammers,  and  the  like  are  essentially  steam  engines 
adapted  to  some  particular  work.  The  various  tools  operated  by  com- 
pressed air,  such  as  drills,  riveters,  pneumatic  hoists,  and  air  brakes, 
belong  to  the  same  general  class  of  machines,  the  main  difference  being 
that  the  working  fluid  is  air  instead  of  steam.  All  are  machines  by 
means  of  which  a  compressible  fluid  does  work  by  virtue  of  a  change  in 
the  internal  condition  of  the  fluid. 

A  machine  such  as  an  air  compressor  is  the  reverse  of  the  engine  in 
that  it  works  upon  a  compressible  fluid  and  puts  it  in  a  condition  in  which 
it  is  able  to  do  work.  A  pump  which  pumps  water  or  other  liquid  is, 
like  the  compressor,  a  machine  for  doing  work  on  a  fluid  but  differs 
from  the  compressor  in  that  the  fluid  upon  which  it  works  is  practically 
incompressible  and  the  pump  merely  changes  the  position  of  the  fluid 
without  producing  any  appreciable  change  in  its  internal  condition. 

In  the  design  of  any  one  of  these  machines  four  elements  must  be 
considered.  First,  the  properties  of  the  vapor,  gas,  or  liquid  with  which 
the  machine  works;  second,  the  kinematics  of  the  machine  itself,  that 
is,  the  geometry  of  the  machine;  third,  the  dynamics  of  the  machine, 
that  is,  the  transmission  of  forces  through  the  parts  of  the  machine; 
fourth,  the  details  of  construction  of  the  parts  so  that  they  shall  be  strong 
enough  and  be  practical  to  make  and  use.  These  four  elements  are, 
of  course,  closely  related  to  each  other  and  it  is  impossible  to  study  one 
without  some  reference  to  the  others. 

In  the  present  treatise  we  are  to  deal  principally  with  the  kinematics 
of  some  of  the  machines  already  referred  to,  touching  upon  the  other 
sides  of  the  question  only  so  far  as  is  necessary  in  order  to  deal  with  the 
subject  in  a  logical  and  intelligent  manner. 

Since  the  steam  engine  is  the  most  common  and  important  of  these 
machines,  and  the  principles  involved  are  broader,  we  shall  consider  that 


Vlll  INTRODUCTION 

first  and  considerably  more  in  detail  than  the  other  machines  which 
follow. 

Steam  engines  may  be  divided  into  two  general  classes: 

1.  Those,  known  as  reciprocating  engines,  in  which  the  steam  im- 
parts a  reciprocating  motion  to  a  piston,  and  this  motion  by  means  of  a 
suitable  mechanism  causes  rotation  of  a  shaft  or  else  is  carried  directly 
from  the  piston  to  the  point  where  the  work  is  done. 

2.  Those  in  which  the  steam  imparts  rotation  to  a  shaft  directly  with- 
out the  intervention  of  a  reciprocating  piston. 


(x) 


MECHANISM  OF 

STEAM   ENGINES 


CHAPTER  I 

GENERAL  DISCUSSION   OF   A  RECIPROCATING   STEAM 

ENGINE 

1.  There  are  a  great  many  types  of  reciprocating  steam  engines,  dif- 
fering widely  in  size  and  general  design.     Certain  fundamental  princi- 
ples of  design  and  method  of  action  are  common  to  all  however.     The 
parts  of  a  reciprocating  engine  may  be  divided  into  three  main  groups: 

1.  Stationary  parts  (frame,  cylinder,  bearings). 

2.  Piston,  piston  rod,  crosshead,  connecting  rod,  crank,  shaft, 

and  flywheel,  to  which  the  steam  imparts  motion. 

3.  Valve  mechanism,  which  controls  the  supply  of  steam. 

The  most  direct  way  to  gain  familiarity  with  the  parts  and  with  the 
principles  of  operation  is  to  study  in  detail  a  simple  example. 

2.  Description  of  a  Simple  Engine.     Fig.  i  represents  a  small  recip- 
rocating engine  of  the  type  known  as  a  plain-slide-valve  engine.'    Directly 
on  the  concrete  or  masonry  foundation  rests  the  frame  D,  carrying,  in 
suitable  bearings  near  one  end,  the  engine  shaft  0,  while  bolted  to  it 
at  the  other  end  is  the  cylinder  E.     The  cylinder  is  closed  at  the  ends 
by  heads,  and  is  covered,  or  lagged,  with  some  material  which  is  a  good 
non-conductor  of  heat,  to  prevent  too  rapid  radiation.     In  the  cylinder 
is  the  piston  F,  which  moves  from  one  end  to  the  other  under  the  influ- 
ence of  steam  pressure.     There  must  be  no  leakage  of  steam  past  the 
piston,  and  it  is  made  steam  tight  by  two  split  rings  in  grooves  around 
the  piston,  which  spring  outward  and  press  against  the  cylinder  walls. 
The  piston  is  rigidly  attached  to  the  piston  rod  G,  the  latter  being  at- 
tached at  the  other  end  to  the  crosshead  H.     Where  the  piston  rod  passes 
through  the  cylinder  head  leakage  of  steam  is  prevented  by  packing. 
The  crosshead  slides  back  and  forth  between  the  guides  /,  which  pre- 
vent any  tendency  to  bend  the  piston  rod. 


2  ' *'  *  'MECHA'NlSM  4  OF  STEAM  ENGINES 

The  motion  of  the  crosshead  is  carried  to  the  crank  pin  A  by  means 
of  the  connecting  rod  /,  the  latter  being  attached  to  the  crosshead  by 
the  wrist  pin  or  crosshead  pin  B.  The  connecting  rod  is  provided  with 
boxes  of  suitable  bearing  metal,  and  provision  is  made  for  taking  up 
wear.  In  this  particular  engine  the  shaft  0,  crank,  and  crank  pin  are 
forged  in  one  piece,  called  a  crank  shaft.  The  weight  of  the  crank  and 
crank  pin  and  part  of  the  weight  of  the  connecting  rod  are  balanced  by 
the  counterweights  K,  which  latter  are  bolted  to  the  crank.  The  shaft 
in  this  case  carries  two  heavy  flywheels,  which  serve  to  make  the  engine 
run  steadily  and  provide  a  means  of  taking  off  power  by  belts.  The 
eccentric  L  is  fast  to  the  shaft  and  is  connected  by  the  eccentric  strap 
and  eccentric  rod  to  the  valve  stem  guide  W  which  in  turn  is  connected 
to  the  valve  V  by  the  valve  stem  Y.  The  valve  has  a  reciprocating 
motion  on  suitable  guides,  in  a  steam-tight  box  C,  known  as  a  steam 
chest,  or  valve  chest,  which  is  cast  on  the  side  of  the  cylinder.  This 
valve  controls  the  flow  of  steam  to  and  from  the  cylinder.  The  vertical 
surface,  against  which  the  valve  runs,  and  which  is  called  the  valve 
seat,  has  in  it  three  openings  M ,  ^V,  and  P  called  ports;  M  and  N  open 
into  the  cylinder  near  the  ends  while  P  connects  to  the  exhaust  pipe  R. 
The  metal  left  between  the  ports  forms  the  bridges.  These  ports  and 
bridges  are  shown  in  Fig.  i  b  and  Fig.  i  d. 

3.  Fundamental  Definitions.    The  end  of  the  cylinder  which  is  nearer 
the  crank  is  usually  spoken  of  as  the  crank  end  while  the  opposite  end 
is  called  the  head  end.     The  port  M  is  called  the  crank-end  steam  port 
while  N  is  called  the  head-end  steam  port;  P  is  called  the  exhaust  port. 
When  the  crank  and  connecting  rod  are  in  line,  the  crank  being  toward 
the  cylinder  and  the  piston  at  the  head  end,  the  engine  is  said  to  be  on 
the  head  end  dead  point  or  dead  center.     After  the  crank  has  turned 
1 80°  so  that  the  piston  is  at  the  crank  end  of  the  cylinder  the  engine 
is  said  to  be  on  the  crank  end  dead  point  or  dead  center.     The  motion 
of  the  piston  from  the  head  end  of  the  cylinder  to  the  crank  end  is  called 
the  forward  stroke,  while  the  motion  from  the  crank  end  back  to  the 
head  end  is  called  the  return  stroke. 

4.  Action  of  the  Engine.    Referring  to  Fig.  i  a,  steam  from  the  boiler 
enters  the  steam  chest  through  the  throttle  valve  T,  surrounds  the  valve 
and  enters  port  N  as  soon  as  the  valve  uncovers  it,  thus  admitting 
steam  on  the  head-end  side  of  the  piston.    At  the  same  time  steam  which 
has  already  done  its  work  on  the  crank-end  side  of  the  piston  may  flow 
out  through  the  port  M,  into  the  exhaust  cavity  of  the  valve,  around  the 


GENERAL  DISCUSSION  OF  A  RECIPROCATING  STEAM  ENGINE        J 

bridge  and  into  the  exhaust  port  P.  Fig.  i  c,  which  is  a  section  through 
cylinder,  steam  chest  and  valve,  will  help  to  make  clearer  how  the  steam 
enters  and  leaves  the  steam  chest.  The  difference  of  pressure  on  the 
two  sides  of  the  piston  drives  it  toward  the  crank  end  of  the  cylinder 
and  its  motion  is  transmitted  through  the  piston  rod,  crosshead  and 
connecting  rod  to  the  crank  pin,  thus  causing  the  shaft  to  turn.  At  the 
proper  time  the  valve  moves  so  as  to  stop  the  flow  of  steam  into  the  head 
end,  then  connects  the  head  end  with  the  exhaust,  and  finally  moves 
far  enough  to  admit  steam  through  port  M  into  the  crank  end,  thus 
driving  the  piston  back  to  the  head  end.  This,  in  brief,  is  the  way  that 
steam  under  pressure  causes  a  piston  to  have  a  reciprocating  motion 
which,  in  turn,  is  transformed  into  a  continuous  rotation  of  the  shaft. 

The  opening  of  the  port  to  admit  steam  to  the  cylinder  is  called  ad- 
mission, cutting  off  the  supply  by  closing  the  port  is  called  cut-o/,  the 
opening  of  the  exhaust  for  spent  steam  is  called  release  and  the  closing 
of  the  exhaust  is  called  compression.  These  are  the  four  events  of  the 
stroke  and  will  be  discussed  in  detail  later. 

For  each  end  of  the  cylinder  the  events  occur  in  this  order:  admission, 
cut-off,  release  and  compression,  and  they  will  be  designated  by  the 
letters  A,  C,  R  and  K,  respectively,  while  subscripts  (k)  and  (c)  will  in- 
dicate to  which  end  the  event  belongs;  thus,  Ah  indicates  admission  on 
the  head  end  while  Kc  indicates  compression  on  the  crank  end.  The 
abbreviations  H.E.  and  C.E.  may  be  used  to  indicate  head  end  and  crank 
end  respectively. 

It  is  important  to  know  approximately  where  the  crank  is  when  each 
of  these  events  occurs  and  what  is  taking  place  in  the  cylinder  in  the 
intervening  time.  This  is  indicated  in  Fig.  2  for  the  head  end,  and  a 
similar  diagram  might  be  drawn  for  the  crank  end,  the  direction  of  rota- 
tion being  as  shown  by  the  arrow.  Head-end  admission  usually  occurs 
just  before  the  crank  reaches  the  head-end  dead  point,  when  the  crank 
is  about  at  Ah\  steam  flows  into  the  cylinder  until  the  crank  reaches 
some  such  position  as  Ch,  when  cut-off  occurs.  The  position  of  Ch  de- 
pends upon  the  way  the  eccentric  and  valve  are  set,  this  setting,  in 
turn,  depending  partly  upon  the  amount  of  work  the  engine  is  doing. 
After  cut-off  the  port  is  closed  for  a  time,  and  the  steam  confined  in  the 
cylinder  forces  the  piston  along  by  expanding.  When  the  crank  pin 
reaches  Rh  the  valve  will  have  moved  so  as  to  begin  to  uncover  the  port 
on  the  exhaust  side  and  allow  the  spent  steam  to  begin  to  flow  out.  It 
continues  to  flow  out  until  the  crank  pin  reaches  Kh,  when  the  port  will 


MECHANISM   OF    STEAM   ENGINES 


close  for  exhaust  and  the  steam  remaining  in  the  cylinder  is  compressed 
ahead  of  the   piston,   thus  serving  to   check  the  momentum    of  the 


FIG.  2. 

reciprocating  parts,  preparatory  to  the  reversal  of  direction  of  piston 
movement,  which  occurs  when  the  crank  reaches  the  dead  point. 

Piston,  Crosshead,  Connecting  Rod  and  Crank 

5.  From  the  preceding  description  it  is  evident  that  the  reciprocating 
motion  of  the  piston  is  transferred  by  the  piston  rod  to  the  crosshead, 
and  is  transformed  into  rotary  motion  of  the  shaft  by  the  connecting  rod 
and  crank.     It  is  important  to  get  clearly  in  mind  the  action  of  these 
parts  and  the  effect  of  the  connecting  rod  on  the  motion  of  the  cross- 
head  and  therefore  of  the  piston  if,  under  the  steadying  action  of  the 
flywheel,  the  shaft  turns  with  uniform  angular  speed.     Since  the  motion 
of  crosshead  and  piston  are  the  same  we  will  refer  to  the  motion  of  the 
crosshead  pin  as  the  motion  of  the  piston. 

6.  Displacement  of  Crosshead.     In  referring  to  the  piston  position 
at  any  time  it  is  customary  to  describe  its  position  by  stating  the  linear 
displacement  of  the  crosshead  from  either  one  of  the  extremes.     This 
displacement  is  commonly  given  in  percentage  of  the  length  of  the  stroke. 
For  example,  if  a  certain  event  occurs  when  the  piston  is  moving  toward 
the  crank  end  and  has  moved  three  quarters  of  the  distance  from  the 
head  end  to  the  crank  end  that  event  is  said  to  occur  at  75  per  cent 
of  the  forward  stroke.     The  crosshead  displacement  for  any  given  crank 
angle,  or  the  crank  angle  for  any  given  crosshead  position,  may  be  found 


GENERAL  DISCUSSION  OF  A  RECIPROCATING   STEAM  ENGINE         5 

graphically  or  may  be  calculated.  For  ordinary  work  the  graphical 
method  is  convenient  and  sufficiently  accurate.  It>  is  well,  however,  to 
be  familiar  also  with  the  analytical  method  and  we  will  accordingly  con- 
sider both. 

In  Fig.  3  the  crank  and  connecting  rod  are  shown  diagrammatically. 
When  the  crank  pin  is  at  A\  the  center  of  the  crosshead  pin  is  at  B\t  on 
the  head-end  dead  point.  When  the  crank  has  turned  to  any  position  OA 
the  center  of  the  crosshead  pin  is  at  B,  found  by  cutting  the  path  of  the 
crosshead  pin  with  an  arc  whose  center  is  A  and  whose  radius  is  the  length 
of  the  connecting  rod  (from  center  of  crank  pin  to  center  of  crosshead 
pin).  The  piston  displacement  from  the  head-end  extreme  position  is 


FIG.  3. 

therefore  M.  If  this  construction  is  drawn  accurately  to  scale  the  dis- 
tance M  can  be  measured  off  accurately.  If  M  is  desired  in  percentage 
of  the  stroke  it  is  only  necessary  to  divide  M  by  the  length  of  the 
stroke.  If  the  crank  is  drawn  at  a  suitable  scale  the  distance  M  can 
be  measured  off  directly  in  percentage  of  the  stroke.  For  example, 
if  OA  is  made  2\"  on  the  drawing  the  stroke  will  measure  5"  or  VF""- 
Accordingly  ^V'  *s  one  Per  cent.  Therefore,  if  M  is  •£-$"  it  will  be  8  per 
cent  of  the  stroke.  Similarly  if  OA  =  i^V,  ^V"  ^s  one  Per  cen^  or  ^ 
OA  =  3-J",  -j1^"  is  one  per  cent.  It  is  advisable,  when  possible,  to  draw 
the  crank  and  connecting  rod  at  one  of  the  above  scales  or  at  some 
similar  scale  so  that  some  convenient  fraction  of  an  inch  is  -j-J-g-  of  the 
stroke. 

If  the  crosshead  displacement  is  known  the  corresponding  crank  posi- 
tion may  be  found  by  locating  the  center  of  the  crosshead  pin  and  from 
this  point,  with  a  radius  equal  to  the  length  of  the  connecting  rod,  cutting 
the  crank  pin  circle. 

The  equation  for  calculating  the  displacement  M  when  the  crank 
position  is  known,  or  for  calculating  the  angle  6  which  the  crank  makes 


MECHANISM  OF  STEAM   ENGINES 


with  the  center  line  when  the  crosshead  displacement  M  is  known,  may 
be  derived  as  follows: 


M  =  OBi  -  OB,  where  OB±  =  OA  +  AB. 
Now  drop  a  perpendicular  from  A  to  the  center  line. 
Then  OB  =  OK  + 


From  triangle  OAK 

OK  =  OA  cos  B. 
From  triangle 


-  O42  sin2  (9. 
Therefore 


OB  =  O^  cos  6  +  V^2  -  O42  sin2  0, 
and 


=  OA  +  ,4£  -  0,4  cos  0  -      ^B2  -  OA2  sin2  6 


=  OA(i-  cos  0)  +  ^fi  -  V  i  -  !=s  sin2  01  .  (i) 


C  is  often  used  to  represent  the  length  of  the  crank  and  L  to  represent 
the  length  of  the  connecting  rod,  -  being,  therefore,  the  ratio  of  the  con- 

Lx 

necting  rod  to  the  crank. 

7.  Velocity  of  Crosshead.  Assuming  that  the  crank  turns  with  uni- 
form angular  speed  the  speed  of  the  crank  pin  is  uniform  and  is  expressed 
by  the  equation 

S  =  27rCN, 

where  S  is  the  speed  of  crank  pin  in  linear  units  per  unit  of  time,  C  the 
length  of  the  crank  (in  the  same  linear  units)  and  N  the  number  of  rev- 
olutions of  the  crank  in  the  unit  time.  N  is  commonly  expressed  in 
revolutions  per  minute  (R.P.M.)  and  C  in  feet,  therefore  S  will  be  in  feet 
per  minute.  The  speed  of  the  crosshead  will  vary  from  zero  at  the  dead 
points  to  a  maximum  at  some  point  between.  The  speed  of  the  cross- 
head  for  any  given  crank  position  may  be  found  graphically  from  the 
speed  of  the  crank  pin  by  any  one  of  several  constructions,  one  of 
which  is  shown  in  Fig.  4. 


GENERAL  DISCUSSION  OF  A  RECIPROCATING   STEAM  ENGINE 


It  can  be  proved  that 

Speed  of  crosshead  _  Om  9 

Speed  of  crank  pin      OA 

where  m  is  the  point  where  the  center  line  of  the  connecting  rod  cuts  the 
line  through  O,  perpendicular  to  the  center  line  of  crosshead  pin  motion. 


FIG.  4. 

For  a  known  speed  of  the  crank  pin  the  crosshead  speed  may  be  found  as 
follows: 

On  the  crank  lay  off  Od  to  represent  the  speed  of  the  crank  pin. 
Through  d  draw  a  line  parallel  to  AB  cutting  at  e  the  line  through  0, 
perpendicular  to  the  line  of  the  crosshead  motion.  Then  Oe  represents 
the  speed  of  B.  The  following  equation  expresses  the  ratio  of  the  speed 
of  the  crosshead  to  the  speed  of  the  crank  pin  for  any  given  crank 
angle  0. 

Csin0cos0  ,  , 


Speed  crosshead 

-  =  sin 


Speed  crank  pin 


VL2-C2sin20 


Hn 


01    £' 


Piston  Rod 


where  C  represents  the  length  of  the  crank  and  L  the  length  of  the  con- 
necting rod. 

The  proof  of  the  above  equation 
as  well  as  of  the  preceding  graphical 
construction  depends  upon  the  ordi- 
nary principles  of  mechanism  and    -| 
need  not  be  given  here. 

8.  Harmonic  Motion.  If  the  crank 
pin  worked  in  a  slot  in  the  cross- 
head,  as  in  Fig.  5,  instead  of  being 
connected  to  the  crosshead  by  a  con- 


qy 
FIG.  5. 


necting  rod,  the  cross  head  would  have  what  is  known  as  harmonic 
motion  if  the  crank  turned  uniformly.  The  displacement  for  any  crank 
angle  6  is  DE  where  E  is  the  foot  of  a  perpendicular  from  the  center  of 


8 


MECHANISM   OF   STEAM   ENGINES 


the  crank  pin  to  the  center  line  of  the  crosshead  path.     The  equation 
for  the  value  of  this  displacement  is 

DE  =  OD  -  OR  =  OA  (i  -  cos  0).  (3) 

The  extent  to  which  the  motion  of  the  crosshead  with  a  connecting  rod 
varies  from  harmonic  motion  depends  upon  the  length  of  the  connecting 
rod  relative  to  the  crank.  If  the  crosshead  in  Fig.  3  had  harmonic  motion 
its  displacement  for  the  crank  angle  there  shown  would  be  AiK.  Its 
actual  displacement  is  B\B  which,  if  the  drawing  were  at  a  large  enough 
scale,  would  be  noticeably  different  from  A\K.  This  variation  may  be 
seen  by  comparing  equations  (i)  and  (3). 
From  (i)  

/  . 2 

Displacement  =  OA  (i  -  cos  0)  +  AB  |~i  -  V  i  -  ;sin2  01. 

From  (3) 

Displacement  =  OA  (i  —  cos  0). 


Evidently  then  the  displacement  of  the  crosshead  with  a  connecting  rod 
varies  from  harmonic  motion  by  the  quantity 


where  OA  is  the  length  of  the  crank  and  AB  the  length  of  the  connecting 
rod.     As  the  connecting  rod  is  increased  in  length  the  value  of  this 


Stroke*\ 


FIG.  6. 

quantity  becomes  less  and  the  motion  of  the  crosshead  becomes  more 
nearly  harmonic.  In  practice  the  ratio  of  connecting  rod  to  crank  varies 
from  4  to  8. 

One  position  in  which  this  variation  is  especially  noticeable  is  the  mid- 
position  of  the  crosshead,  that  is  the  half  stroke  position.     In  Fig.  6  the 


GENERAL  DISCUSSION   OF  A   RECIPROCATING   STEAM   ENGINE         9 

crosshead  pin  B  is  in  the  middle  of  its  stroke  and  with  the  connecting 
rod  AB  the  crank  pin  is  either  at  A  or  A$  according  as  the  piston  is  on  its 
forward  or  return  stroke.  If  the  motion  were  harmonic  the  crank  pin 
would  be  at  A2  or  A±  when  the  crosshead  is  in  the  middle  of  its  stroke. 
The  shorter  the  connecting  rod  relative  to  the  crank,  the  greater  will  be 
the  angles  AOA2  and  A5OA±.  In  any  case  AOAi  =  AbOA^. 

Valve,  Eccentric  Rod  and  Eccentric 

9.  The  valve,  valve  stem  guide,  eccentric  rod  and  eccentric  constitute 
essentially  the  same  kind  of  a  mechanism  as  the  piston,  crosshead  con- 
necting rod  and  crank;  therefore  the  same  methods  may  be  used  for  deter- 
mining displacements  and  velocities.  There  are  certain  special  features 
about  the  eccentric  mechanism,  however,  which  need  to  be  mentioned. 


FIG.  7. 

Fig.  7  is  a  drawing  of  the  valve  mechanism  for  the  engine  shown  in 
Fig.  i  and  the  names  of  the  parts  have  already  been  given. 

Fig.  8  is  a  diagram  of  an  eccentric,  eccentric  rod  and  valve  stem  guide 
mechanism.  The  eccentric  itself  is  a  circular  disk  keyed  to  the  shaft. 
E  is  the  center  of  the  eccentric  and  0  is  the  center  of  the  shaft.  The 
eccentric  is  really  a  crank  pin  large  enough  to  include  the  shaft.  The 
distance  OE  is  known  as  the  eccentricity  and  corresponds  to  the  distance 
from  the  center  of  the  shaft  to  the  center  of  the  crank  pin  on  a  crank  of 
the  ordinary  kind.  In  our  diagrams  we  will  usually  represent  the  eccen- 
tric by  the  center  line  of  the  equivalent  crank.  When  E  is  in  the  posi- 
tion EI  the  center  D  of  the  pin  which  attaches  the  eccentric  rod  to  the 
valve  stem  is  at  one  end  of  its  travel  and  when  E  is  at  E^  D  is  at  the 
other  end  of  its  travel.  The  total  travel  of  the  valve  therefore,  when 


10  MECHANISM  OF  STEAM   ENGINES 

connected  directly  to  the  eccentric  rod  as  in  this  case,  is  twice  the  eccen- 
tricity. 

10.  Mid-position.     The  term  mid-position  is  one  which  occurs  fre- 
quently in  discussing  valve  movements  and  an  explanation  of  the  meaning 
of  the  term  is  desirable.     In  general  the  valve  is  said  to  be  in  mid-posi- 
tion when  it  is  halfway  between  the  two  extreme  positions  of  its  motion. 
With  certain  special  types  of  valves  which  will  be  considered  later  the 
so-called  mid-position  may  be  chosen  in  some  other  place. 

11.  The  position  of  the  valve  is  usually  described  by  stating  its  dis- 
placement from  mid-position  instead  of  from  one  extreme,  as  was  the 
case  with  the  piston.     For  example,  in  Fig.  8,  since  the  valve  displace- 
ment is,  of  course,  equal  to  the  displacement  of  the  point  D,  the  valve 
would  be  said  to  be  displaced  a  distance  N  toward  the  head  end.     If 


this  displacement  were  to  be  calculated  an  equation  similar  to  equation 
(i)  would  be  used  which  would  give  the  displacement  from  one  end  of 
the  stroke  of  D;  then  the  difference  between  that  calculated  distance  and 
one  half  of  the  stroke  would  give  the  displacement  of  D  from  mid-posi- 
tion. 

As  a  rule  the  eccentric  rod  is  long  relative  to  the  eccentricity  so  that 
the  motion  of  a  direct-connected  valve,  such  as  we  have  been  discussing, 
is  approximately  harmonic.  This  appears  in  Fig.  8  where  the  eccentric 
center  line  OEm  is  nearly  vertical  when  D  is  in  mid-position. 

12.  Rockers.  It  happens  very  frequently  that  the  construction  of 
an  engine  is  such  that  the  valve  rod  cannot  be  directly  connected  to  the 
eccentric  rod.  It  then  becomes  necessary  to  interpose  some  device  to 
transfer  the  motion  from  one  line  to  another,  in  the  same  plane,  or  fre- 
quently from  one  line  to  another  in  different  planes.  Figs.  9,  10  and 
ii  show  a  common  device  for  this  purpose,  known  under  the  general 
name  of  rocker.  In  Fig.  9  the  axis  or  fulcrum  is  at  A,  the  eccentric  rod 
attaches  at  B  and  the  valve  stem  at  C.  Since  B  and  C  are  on  the  same 
side  of  the  pivot  the  direction  of  motion  of  the  valve  stem  is  not  changed 
by  the  rocker.  The  total  travel  of  the  valve  is  practically  equal  to  the 


GENERAL   DISCUSSION   OF  A   RECIPROCATING   STEAM   ENGINE       II 

Valve  Stem 


FIG.  9. 


C      Valve  Stem 


FIG.  10. 


FIG.  ii. 


12 


MECHANISM   OF   STEAM   ENGINES 


AC 


eccentricity  multiplied  by  —  —  .     In  Fig.   10  the  same  general  type  of 


rocker  is  shown  but  here  the  arms  AB  and  AC  do  not  coincide  so  that 
the  valve  travel  is  not  necessarily  equal  to  the  eccentricity  multiplied 
by  the  ratio  of  the  arms.  Rockers  of  the  kind  shown  in  these  two  figures 
are  sometimes  called  bell  crank  levers.  The  term  carrier  is  also  some- 
times applied  to  them. 

In  Fig.  ii  is  shown  a  rocker  in  which  the  pivot  is  between  the  points  of 
attachment  of  the  eccentric  rod  and  valve  stem.  The  points  B  and  C 
are  evidently  at  all  times  moving  in  opposite  directions.  Rockers  of 
this  type  may  be  made  with  the  arms  180°  apart  or  at  some  less  angle. 
If  the  angle  is  180°  the  valve  travel  is  practically  equal  to  the  eccentricity 
multiplied  by  the  ratio  of  the  arms,  while  with  an  angle  other  than  180° 
this  is  not  necessarily  true.  For  convenience  we  will  refer  to  rockers 
of  the  style  shown  in  Figs.  9  and  10  as  non-reversing  rockers,  and  those 
in  which  the  pivot  is  between  the  eccentric  rod  pin  and  valve-stem  pin 
as  reversing  rockers. 

The  arms  of  any  rocker  may  lie  in  the  same  or  in  different  planes. 

The  methods  for  designing  rockers  and  for  setting  the  eccentric  to 
give  proper  motion  to  the  valve  when  driving  through  the  various  kinds 
of  rockers  will  be  referred  to  later. 


'Line  of  Atmospheric  Pressure 
Abscissas  =  Piston  Displacements 
FIG.  12. 

13.  Indicator  Diagrams.  In  studying  the  action  of  a  steam  engine, 
particularly  as  related  to  the  design  and  adjustment  of  the  valve  gear, 
it  is  convenient  to  have  some  sort  of  a  diagram  which  shall  show  how 
the  steam  is  distributed  during  a  revolution. 


GENERAL  DISCUSSION  OF  A  RECIPROCATING   STEAM  ENGINE      13 

The  conditions  within  the  cylinder  may  be  represented  by  a  diagram, 
such  as  that  shown  in  Fig.  12.  The  abscissae  are  piston  displacements, 
at  some  known  scale,  and  the  ordinates  are  corresponding  pressures  in 
the  cylinder  measured  in  pounds  per  square  inch  and  plotted  at  a  con- 
venient scale.  In  Fig.  12,1  inch  represents  60  pounds  pressure  per  square 
inch,  and  a  distance  of  4  inches  represents  the  length  of  the  stroke. 

In  practice  the  engine  is  made  to  draw  its  own  diagram  by  means  of 
the  device  shown  in  Figs.  13  and  14,  known  as  an  engine  indicator. 


FIG.  13.     Crosby  Steam  Engine  Indicator. 

These  figures  and  the  following  description  taken  from  a  book  published 
by  the  Crosby  Steam  Gage  and  Valve  Company  will  explain  the  action 
of  the  indicator. 

"A  piston  of  carefully  determined  area  is  nicely  fitted  into  a  cylinder 
so  that  it  will  move  up  and  down  without  sensible  friction.  The  cylinder 
is  open  at  the  bottom  and  fitted  so  that  it  may  be  attached  to  the  cylin- 
der of  a  steam  engine  and  have  free  communication  with  its  interior,  by 


14  MECHANISM  OF  STEAM  ENGINES 

which  arrangement  the  under  side  of  the  piston  is  subjected  to  the  vary- 
ing pressure  of  the  steam  acting  therein.  The  upward  movement  of 
the  piston  —  due  to  the  pressure  of  the  steam  —  is  resisted  by  a  spiral 
.spring  of  known  resilience.  A  piston  rod  projects  upward  through  the 
cylinder  cap  and  moves  a  lever  having  at  its  free  end  a  pencil  point, 
whose  vertical  movement  bears  a  constant  ratio  to  that  of  the  piston. 
A  drum  of  cylindrical  form  and  covered  with  paper  is  attached  to  the 
cylinder  in  such  a  manner  that  the  pencil  point  may  be  brought  in  con- 


FIG.  14.     Section  of  Crosby  Indicator. 

tact  with  its  surface,  and  thus  record  any  movement  of  either  paper  or 
pencil;  the  drum  is  given  a  horizontal  motion  coincident  with  and  bear- 
ing a  constant  ratio  to  the  movement  of  the  piston  of  the  engine.  It  is 
moved  in  one  direction  by  means  of  a  cord  attached  to  the  crosshead 
and  in  the  opposite  direction  by  a  spring  within  itself. 

"When  this  mechanism  is  properly  adjusted  and  free  communication 
is  opened  with  the  cylinder  of  a  steam  engine  in  motion,  it  is  evident 


GENERAL  DISCUSSION  OF  A  RECIPROCATING  STEAM  ENGINE      15 

that  the  pencil  will  be  moved  vertically  by  the  varying  pressure  of  steam 
under  the  piston,  and  as  the  drum  is  rotated  by  the  reciprocating  motion 
of  the  engine,  if  the  pencil  is  held  in  contact  with  the  moving  paper 
during  one  revolution  of  the  engine,  a  figure  or  diagram  will  be  traced 
representing  the  pressure  of  steam  in  the  cylinder,  the  upper  line  show- 
ing the  pressure  urging  the  engine  piston  forward,  and  the  lower  the 
pressure  retarding  its  movement  on  the  return  stroke. 

"To  enable  the  engineer  to  more  correctly  interpret  the  nature  of 
the  pressures,  the  line  showing  the  atmospheric  pressure  is  drawn  in  its 
relative  position,  which  indicates  whether  the  pressure  at  any  part  is 
greater  or  less  than  that  of  the  atmosphere. 

"From  such  a  diagram  may  be  deduced  many  particulars  which  are 
of  supreme  importance  to  engine  builders,  engineers,  and  the  owners  of 
steam  plants. " 

The  diagram  is  called  an  indicator  card  and  is  useful  in  determining 
the  time  at  which  the  events  of  the  stroke  occur  and  also  in  getting  an 
idea  of  the  power  of  the  engine. 

Referring  again  to  Fig.  12,  which  is  an  indicator  card  for  the  head  end, 
when  the  piston  is  at  any  point  M  the  pressure  in  the  cylinder  is  repre- 
sented by  the  length  of  the  line  MN.  The  scale  of  ordinates  on  this 
particular  diagram  is  i"  =  60  pounds.  The  line  MN  measures  |-|"; 
therefore  the  pressure  at  that  time  is  73  pounds  per  square  inch.  The 
ordinate  at  B  shows  the  pressure  at  the  beginning  of  the  stroke,  that  at 
C  the  pressure  at  cut-off,  the  ordinate  at  R  the  pressure  at  release,  the 
ordinate  at  K,  the  pressure  at  beginning  of  compression,  and  the  ordinate 
at  A  the  pressure  at  admission.  On  a  card  drawn  by  an  indicator  the 
points  A,  B,  C,  R  and  K  can  be  determined  by  inspection  and  therefore 
the  corresponding  piston  position  can  be  found.  The  average  of  all  the 
ordinates  of  the  upper  part  of  the  curve  will  give  the  average  pressure 
exerted  by  the  steam  on  the  head  end  of  the  piston  during  the  forward 
stroke.  This  average  pressure  multiplied  by  the  area  of  the  piston  in 
square  inches  and  by  the  length  of  the  stroke  in  feet  will  give  the  foot 
pounds  of  work  done  by  the  steam  on  the  head  end  of  piston  during  that 
stroke.  In  a  similar  way  the  average  of  all  the  ordinates  of  the  lower 
part  of  the  curve  will  give  the  average  pressure  acting  against  the  head 
end  of  the  piston  on  the  return  stroke  and  this  multiplied  by  the  piston 
area  in  inches  and  length  of  the  stroke  in  feet  will  give  the  foot  pounds 
of  work  which  the  piston  is  obliged  to  do  on  the  steam  in  the  head  end 
of  the  cylinder  in  order  to  make  the  return  stroke.  The  difference  of  the 


1 6  MECHANISM  OF   STEAM   ENGINES 

work  done  by  the  steam  during  the  forward  stroke  and  the  work  done 
against  the  steam  during  the  return  stroke  will  be  the  net  work  done 
by  the  steam  on  the  head  end  of  the  piston  during  a  complete  revolution 
of  the  crank.  The  work  done  in  the  crank  end  can  be  found  in  the  same 
way.  The  sum  of  the  two  will  be  the  total  work  done  on  the  piston 
during  one  revolution.  This  multiplied  by  the  number  of  revolutions 
per  minute  and  divided  by  33,000  will  give  the  horse  power  of  the 
engine. 

In  actually  making  use  of  the  card  to  find  the  power  which  the  engine 
is  developing  the  following  method  is  commonly  used.  The  area  enclosed 
by  the  curve  ABCRK  is  measured  in  square  inches  by  an  instrument 
called  a  planimeter.  This  area  divided  by  the  length  of  the  card  in 
inches  will  give  the  average  of  all  the  ordinates  which  represent  net  effec- 
tive pressures,  that  is,  the  average  of  all  the  ordinates  for  the  upper 
curve  minus  the  average  of  the  ordinates  on  the  lower  curve.  This 
average  net  length  of  ordinate  multiplied  by  the  scale  of  the  indicator 
spring  gives  the  average  effective  pressure  per  square  inch  of  piston 
area  which  is  available  throughout  a  complete  revolution  for  work  on 
that  side  of  the  piston.  The  name  mean  effective  pressure  (abbreviated 
M.E.P.)  is  commonly  given  to  this  quantity.  Then  M.E.P.  X  area  of 
piston  in  square  inches  X  length  of  stroke  in  feet  X  R.P.M.  -r-  33,000  = 
H.P.  indicated  by  this  card.  The  sum  of  this  result  and  the  corre- 
sponding one  for  the  other  end  will  give  what  is  known  as  the  indicated 
horse  power  of  the  engine  (I.H.P.). 

14.  Types  of  Engines.  Reciprocating  steam  engines  may  be  class- 
ified in  a  variety  of  ways,  some  of  which  are  as  follows: 

1.  By  position: 

(a)  Horizontal. 
(&)  Vertical. 

2.  By  expansion  of  steam: 

(a)  Simple. 

Cross  compound. 


(b)  Compound 


Tandem  compound. 


Angle  compound. 
(c)  Multiple-expansion. 
By  disposal  of  exhaust: 

(a)  Non-condensing. 

(b)  Condensing. 

By  kind  of  valve  or  valve  mechanism. 


GENERAL  DISCUSSION  OF  A  RECIPROCATING   STEAM   ENGINE       17 

15.  Horizontal  and  Vertical  Engines.     In  a  horizontal  engine  the  axis 
of  the  cylinder  is  a  horizontal  line  while  in  a  vertical  engine  it  is  a  vertical 
line.     In  both  cases  the  axis  usually  intersects  the  axis  of  the  shaft. 
Fig.  15  illustrates  a  simple  horizontal  engine  and  Fig.  16  a  simple  vertical 
engine. 

16.  Simple,  Compound  and  Multiple-expansion  Engines.     The  steam 
may  do  all  of  its  work  in  one  cylinder,  entering  at  practically  boiler 
pressure  and  expanding  down  to  the  pressure  at  which  it  is  delivered  to 
the  exhaust  pipe,  or  it  may  do  a  part  of  its  work  in  one  cylinder, 
then  exhaust  directly  into  a  second  cylinder  in  which  it  does  more  work 
and  from  which  it  goes  to  the  exhaust  pipe,  or  the  second  cylinder  may 
exhaust  into  a  third.     Even  four  or  more  are  possible  although  not  com- 
mon.    An  engine  in  which  all  the  expansion  of  the  steam  takes  place  in 
one  cylinder  is  known  as  a  simple  engine.     Both  of  those  in  Figs.  15  and 
1 6  belong  to  this  class. 

An  engine  in  which  a  second  cylinder  makes  use  of  the  exhaust  from 
the  first  is  called  a  compound  engine.  Fig.  17  shows  a  compound  engine, 
of  the  kind  known  as  cross  compound,  in  which  the  two  cylinders  are  side 
by  side,  each  piston  being  connected  to  its  own  crank  on  the  shaft.  In 
the  figure  the  cylinders  themselves  do  not  show  to  any  extent  as  they 
are  enclosed  in  the  rectangular  casings.  It  is  evident,  however,  that 
the  nearer  cylinder,  which  is  the  "high  pressure,"  is  smaller  than  the 
other.  This  is  to  make  allowance  for  the  difference  in  steam  pressures  as 
it  is  desirable  to  distribute  the  work  equally  between  the  two.  The  box- 
like  structure  at  the  left  of  each  cylinder  encloses  a  rod  known  as  a  tail 
rod,  which  is  really  an  extension  of  the  piston  rod  running  through  the 
outer  cylinder  head.  The  tail  rod  helps  support  the  weight  of  the 
piston  so  that  the  weight  does  not  come  on  the  cylinder  walls.  This 
feature  is  absent  on  nearly  all  small  engines  and  on  many  larger  ones. 
The  exhaust  from  the  high-pressure  cylinder  is  delivered  either  directly 
into  the  supply  pipe  for  the  low-pressure  cylinder  or  else  into  a  closed 
chamber,  known  as  a  receiver,  from  which  the  "low"  draws  its 
supply. 

Fig.  1 8  shows  a  tandem  compound  in  which  the  two  cylinders  are  in 
line,  both  pistons  being  on  the  same  piston  rod.  The  pipe  which  carries 
the  exhaust  from  the  high-pressure  cylinder  to  the  low  is  plainly  seen  in 
this  picture. 

In  Fig.  19  is  shown  an  angle  compound  engine  where  the  high-pressure 
cylinder  is  horizontal  and  the  low-pressure  cylinder  vertical. 


i8 


MECHANISM  OF  STEAM  ENGINES 


GENERAL  DISCUSSION  OF  A  RECIPROCATING   STEAM   ENGINE       19 


FIG.  16.    Simple  Vertical  Engine.     Buckeye  Engine  Co. 


2O 


MECHANISM   OF   STEAM   ENGINES 


GENERAL  DISCUSSION  OF  A   RECIPROCATING   STEAM   ENGINE      21 


w 


22 


MECHANISM   OF  STEAM  ENGINES 


Fig.  20  shows  a  vertical  engine  in  which  the  steam  expands  through 
three  cylinders,  known  respectively  as  the  high,  intermediate  and  low. 
Such  an  engine  is  called  triple  expansion. 

17.  Non-condensing  and  Condensing  Engines.  The  exhaust  steam 
may  be  discharged  directly  into  the  atmosphere  or  it  may  be  discharged 


FIG.  19.    Angle-compound  Engine.    American  Engine  &  Electric  Co. 

into  some  sort  of  a  closed  chamber  in  which  a  partial  vacuum  is  main- 
tained and  in  which  the  steam  is  condensed  by  means  of  cold  water. 
The  condensed  steam  is  then  returned  to  the  boiler.  The  engine  which 
exhausts  into  the  atmosphere  is  said  to  be  non-condensing  and  the  other 
class  condensing. 

18.   There  is  a  great  variety  of  valves  and  valve  gears  used  to  control 
the  steam  supply  from  the  steam  chest  to  the  cylinder.     Using  the  kind 


GENERAL  DISCUSSION  OF  A   RECIPROCATING  STEAM   ENGINE      23 


UU 

I 


24  MECHANISM   OF   STEAM   ENGINES 

of  valve  as  a  basis,  stationary  engines  may  be  roughly  divided  into  three 
classes. 

1.  Single- valve  engines  in  which  a  single  valve  controls  all  the  events, 

as  in  the  case  of  the  engine  described  in  §  2. 

2.  Double- valve  engines,  in  which  one  valve  controls  the  admission 

and  the  exhaust  events,  and  an  auxiliary  valve  is  used  to  control 
the  cut-off. 

3.  Multiple- valve  engines,  which  have  three  or  more  valves. 

In  the  case  of  locomotive  engines,  marine  engines  and  certain  others, 
it  is  necessary  to  so  arrange  the  valve  gear  that  the  direction  of  rotation 
may  be  quickly  reversed.  Such  engines  are  known  as  reversing  engines, 
and  the  mechanism  driving  the  valve  is  described  as  a  reversing  gear. 

19.  The  relative  advantages  of  the  various  types  which  have  been 
mentioned  cannot  properly  be  discussed  at  this  time.  What  has  been 
said,  however,  will  serve  to  acquaint  the  student  with  the  fact  that  the 
various  types  exist  and  will  permit  of  reference  being  made  to  them  inci- 
dentally as  may  seem  necessary  in  considering  the  different  questions 
connected  with  the  mechanism  of  the  engines. 


CHAPTER  II 
SINGLE-VALVE   ENGINES 

20.  The  general  arrangement  of  the  valve  gear  for  a  simple  single- valve 
engine  has  been  shown  in  Fig.  i.     The  valve  there  shown  is  a  plain-slide 
or  D  valve.     Fig.  21  is  a  perspective  drawing  of  a  similar  valve  and 
Fig.  22  gives  three  orthographic  views  of  the  same.     This  is  the  simplest 
form  of  single  valve  and  although  not  so  much  used  at  the  present  time 
on  important  engines  as  some  of  the  more  complicated  forms  of  valves, 
the  fundamental  principles  of  design  and  action  can  be  more  clearly 
understood  from  the  simple  valve. 

21.  Laps.     The  valve  is  ordinarily  made  of  such  dimensions  that  it 
overlaps  the  edges  of  the  ports  when  in  mid-position,   as   shown  in 
Fig.   23.     The  amounts  by  which  the  edges  of  the  valve  overlap  the 
corresponding  edges  of  the  ports  when  in  mid-position  are  called  the 
laps.    In  Fig.   23,  Lh  is  the  head-end   steam   lap,  Lc  the   crank-end 
steam  lap,  Nh  the  head-end  exhaust  lap  and  Ne  the  crank-end  exhaust, 
lap. 

The  steam  laps  are  always  positive  quantities  and  may  or  may  not  be 
equal.  The  exhaust  laps  may  be  positive,  as  in  Fig.  23,  or  zero,  as  in 
Fig.  24,  or  negative,  as  in  Fig.  25.  A  negative  exhaust  lap  like  Nh, 
Fig.  25,  is  spoken  of  as  an  exhaust  clearance.  It  not  infrequently  happens 
that  the  valve  needs  to  be  designed  with  an  exhaust  lap  at  one  end  and 
exhaust  clearance  at  the  other. 

22.  Lead  and  Lead  Angle.     Experience  has  shown  that  it  is  often 
desirable  to  have  the  admission  of  steam  occur  just  before  the  piston 
reaches  the  end  of  the  stroke,  so  that  when  the  crank  reaches  the  dead 
point  the  valve  will  have  opened  the  port  by  a  small  amount  for  admis- 
sion of  steam.     The  amount  by  which  the  port  is  open  at  that  time  is 
called  the  lead,  designated  as  head-end  lead  and  crank-end  lead  respec- 
tively, according  as  the  crank  is  on  the  head-end  or  crank-end  dead  point. 

The  angle  which  the  crank  makes  with  the  dead  point  position  when 
admission  occurs  is  called  the  lead  angle. 

25 


26 


MECHANISM   OF   STEAM   ENGINES 


FIG.  21. 


FIG.  22. 


O.E. 


FIG.  23. 


3  H.E. 


FIG.  24. 


SINGLE- VALVE   ENGINES  27 

23.  Angle  Between  Crank  and  Eccentric.     The  position  of  the  eccen- 
tric relative  to  the  crank  depends  upon  a  number, of  conditions,  such  as 
type  of  valve,  connection  between  eccentric  and  valve,  and  time  of  ad- 
mission and  cut-off. 

The  general  statement  may  be  made,  however,  that  the  eccentric 
should  be  so  set  relative  to  the  crank  that  when  the  crank  is  at  the 
proper  place  for  admission  to  occur  the  valve  will  be  displaced  the  proper 
amount  to  be  just  uncovering  the  port  and  moving  in  the  proper  direc- 
tion to  open  the  port. 

24.  Angular  Advance  is  a  term  often  used  in  text-books  and  in  discus- 
sions of  valve  gears.     It  signifies  the  angle  through  which  the  eccentric 
must  be  turned  to  move  the  valve  from  mid-position  a  distance  sufficient 
to  open  the  port  by  an  amount  equal  to  the  lead.     An  equivalent  defi- 
nition is  the  angle  which  the  crank  makes  with  the  dead  point  position 
when  the  valve  is  in  mid-position. 

25.  Position  of  the  Mechanism  for  the  Different  Events  of  the  Stroke. 
Figs.  26  to  32  show  the  valve  mechanism  and  the  reciprocating  parts 
(piston,  crosshead,  connecting  rod  and  crank)  in  the  positions  which 
they  occupy  for  the  several  head-end  conditions.     In  Fig.  26  head-end 
admission  is  just  taking  place;    the  piston  is  nearly   at  the  head-end 
dead  point  position.     The  valve  is  displaced  toward  the  crank  end  an 
amount  equal  to  the  head-end  steam  lap  and  is  moving  toward  the  crank 
end. 

In  Fig.  27  the  piston  is  at  the  head  end  of  its  stroke  and  the  valve  has 
moved  a  little  farther  toward  the  crank  end,  causing  a  small  lead  open- 
ing for  steam  to  flow  into  the  head  end  of  the  cylinder. 

Fig.  28  shows  the  mechanisms  when  the  valve  is  displaced  toward 
the  crank  end  its  maximum  amount  and  the  head-end  port  has  its  max- 
imum opening  for  admission  of  steam. 

Fig.  29  head-end  cut-off  is  just  taking  place.  It  should  be  noted  there 
that  the  valve  is  in  the  same  place  as  when  admission  was  beginning, 
(Fig.  26)  but  is  moving  in  the  opposite  direction. 

Fig.  30  shows  the  mechanisms  when  the  valve  is  in  mid-position, 
moving  toward  the  head  end. 

Fig.  31  shows  the  position  for  head-end  release  and  Fig.  32  for  head-end 
compression.  Here  again  it  is  to  be  noted  that  the  valve  is  in  the  same 
position  in  each  case,  but  is  moving  to  open  the  exhaust  in  Fig.  31  and 
to  close  it  in  Fig.  32.  A  similar  series  of  diagrams  could,  of  course,  be 
drawn  for  the  crank  end. 


MECHANISM  OF  STEAM   ENGINES 


Egocentric  Rod 
Connecting  Rod 


FIG.  26.    Head-end  Admission. 


FIG.  27.    Head-end  Dead  Point. 


FIG.  28.    Extreme  Valve  Displacement. 


SINGLE-VALVE  ENGINES 


29 


FIG.  29.    Head-end  Cut-off. 


FIG.  30.     Mid-position. 


FIG.  31.    Head-end  Release. 


3° 


MECHANISM  OF  STEAM  ENGINES 


26.   A   study  of   the  Figs.   26  to  32  will  make  clear  the  following 
facts: 

1.  When  the  engine  is  at  admission  the  valve  is  displaced  an  amount 

equal  to  the  steam  lap  and  moving  in  the  direction  to  uncover 
the  port. 

2.  When  the  engine  is  at  cut-off  the  valve  is  in  the  same  position 

as  at  admission  for  the  same  end  but  moving  in  the  direction 
to  cover  the  port. 

3.  When  the  engine  is  at  release  the  valve  is  displaced  an  amount 

equal  to  the  exhaust  lap  and  moving  to  uncover  the  port. 

4.  When  the  engine  is  at  the  beginning  of  compression  the  valve 

is  in  the  same  position  as  at  release  for  the  same  end  but  mov- 
ing to  cover  the  port. 


FIG.  32.     Head-end  Compression. 

27.  Equal  Events.     When  the  valve  mechanism  is  so  designed  that 
the  lead  is  the  same  on  both  the  head  and  crank  ends  the  engine  is  said 
to  have  equal  leads.     When  head-end  cut-off  occurs  at  the  same  per- 
centage of  the  forward  stroke  as  does  crank-end  cut-off  of  the  return 
stroke  the  cut-offs  are  said  to  be  equal.     If  compression  occurs  at  the 
same  percentage  of  the  stroke  on  both  strokes  the  compressions  are  equal 
and  similarly  with  reference  to  the  release. 

28.  The  following  statements  may  well  be  made  at  this  time  although 
their  full  force  may  not  be  apparent  until  a  study  is  made  of  valve  dia- 
grams and  problems  in  Chapters  III  and  IV.     These  statements  are  in- 
tended to  apply  to  plain  slide  valves  which  have  approximately  harmonic 
motion,  on  engines  where  the  ratio  of  connecting  rod  to  crank  is  such 
that  the  piston  does  not  have  harmonic  motion. 


SINGLE-VALVE   ENGINES  31 

1.  If  the  leads  are  equal  the  steam  laps  must  be  equal  and  the 

cut-offs  will  be  unequal.  9 

2.  If  the  cut-offs  are  equal  the  steam  laps  will  be  unequal  and 

therefore  the  leads  will  be  unequal. 

3.  Equal  releases  or  equal  compressions  require  unequal  exhaust 

laps. 

4.  If  the  valve  has  a  positive  exhaust  lap  release  will  occur  after 

the  valve  has  passed  mid-position  and  compression  will  occur 
before  the  valve  again  reaches  mid-position. 

5.  If  the  valve  has  an  exhaust  clearance  release  will  occur  before  the 

valve  reaches  mid-position  and  compression  will  not  occur 
until  after  the  valve  again  passes  mid-position  in  the  reverse 
direction. 

29.  Modification  of  the  Slide  Valve.     In  many  cases  valves  are  used 
which,  while  they  are  slide  valves  and  governed  by  the  same  principles, 
are  modified  in  various  ways.     We  will  now  consider  a  few  of  these 
modifications.     The   examples  mentioned   are   chosen   chiefly  because 
they  will  illustrate  the  types  which  they  represent,  no  attempt  being 
made  to  cover  the  entire  field. 

30.  Piston  Valve.     Fig.  33   shows  a  simple  form  of  piston  valve. 
This  is  essentially  a  plain  slide  valve  except  that  it  is  cylindrical,  fitting 
nicely  into  a  cylindrical  chest.     The  ports  spread  out  around  the  chest 
so  that  steam  is  admitted  or  exhausted  around  practically  the  entire 
circumference. 

A  piston  valve  of  a  little  more  complicated  construction  is  shown  in 
Fig.  34.  This  is  a  large  valve  for  use  on  a  locomotive  and  is  provided 
with  packing  rings  to  prevent  leakage.  Here  the  valve  chest  is  fitted 
with  liners,  a  drawing  of  which  is  given  in  Fig.  35. 

31.  Balanced  Valves.     A  plain  D  valve  has  the  full  steam-chest  pres- 
sure on  its  entire  outer  surface  while  its  inner  surface  is  either  in  con- 
tact with  the  seat  or  subjected  only  to  exhaust  pressure,  except  possibly 
the  small  area  which  may  be  over  the  ports  and  subject  to  whatever 
pressure  is  in  the  cylinder.     The  result  is  a  heavy  unbalanced  pressure, 
forcing  the  valve  against  its  seat.     This  means  a  heavy  friction  load  on 
the  gear  which  moves  the  valve. 

The  piston  valve^  is  not  open  to  this  objection  since  the  pressure  on 
it  is  equal  all  around  the  circumference.  There  is  greater  liability 
of  leakage  past  a  piston  valve,  however,  especially  after  it  has  become 
worn,  or  has  worn  the  seat  non-cylindrical. 


MECHANISM  OF  STEAM  ENGINES 


FIG.  33.    Simple  Piston  Valve. 


FIG.  34.     Locomotive  Piston  Valve. 


FIG.  35.     Bushing  for  Piston-valve  Seat. 


SINGLE-VALVE   ENGINES 


33 


Various  devices  have  been  used  to  relieve  a  portion  of  the  unbalanced 
pressure  on  the  flat  seated  valve.  * 

An  example  of  a  balanced  valve  is  shown  in  Fig.  36.  A  pressure 
plate  P  is  bolted  to  the  steam-chest  cover.  A  piece  R  fits  into  a  groove 


FIG.  36.     Locomotive  Balanced  Valve. 

in  the  top  of  the  valve  and  is  held  by  flat  springs  against  the  finished 
under  surface  of  the  pressure  plate  so  as  to  make  practically  a  steam- 
tight  joint  and  thus  shut  out  the  high-steam  pressure  from  the  space  S 


m 


rn 


FIG.  37.     Skinner  Balanced  Valve. 

enclosed  within  it.  R  may  be  a  circular  ring  or  a  rectangular  frame.  A 
small  hole  connects  the  space  S  with  the  exhaust  cavity  so  that  the  pres- 
sure within  S  is  equal  to  the  exhaust  pressure. 

Another  method  of  balancing  is  illustrated  by  the  valve  in  Fig.  37. 
The  valve  is  open  through  the  center  and  has  a  cylindrical  hub  on  its 


34 


MECHANISM  OF  STEAM  ENGINES 


back.  In  this  hub  are  grooves  into  which  fit  packing  rings.  Over  the 
hub  fits  a  sleeve,  and  the  packing  rings  form  a  steam  tight  joint  between 
the  inside  of  the  sleeve  and  the  hub.  The  outer  end  of  the  sleeve  rests 
against  the  finished  inner  surface  of  the  steam-chest  cover,  sliding  with 
the  valve  and  held  tightly  against  the  cover  by  four  helical  springs,  one 
of  which  shows  in  the  drawing.  In  this  way  the  area  of  the  valve  ex- 
posed to  high  steam  pressure  is  reduced  and  the  force  required  to  move 
the  valve  is  made  less. 

Figs.  38  and  39  represent  a  balanced  valve  used  on  the  Ball  engine. 
The  valve  is  a  double  affair,  having  an  upper  and  lower  face  which  are 
alike.  The  lower  part  has  a  hub  which  is  hollow,  while  the  upper  part 


FIG.  38.    American-Ball  Engine  Valve. 


FIG.  39.    American-Ball  Engine  Cylinder 
and  Steam  Chest. 


has  a  hub  which  fits  into  the  lower  one  and  the  fit  is  made  steam  tight 
by  spring  packing  rings.  Thus  the  two  parts  may  telescope  into  each 
other  without  leakage.  The  method  of  connecting  the  stem  to  the  valve 
is  clearly  shown  in  Fig.  38.  A  part  of  the  engine  in  which  this  valve  is 
used  is  seen  in  Fig.  39,  where  the  steam-chest  cover  is  removed.  It 
will  be  seen  that  each  part  of  the  valve  has  its  own  seat  and  ports.  The 
ports  from  the  upper  and  lower  seats  unite  and  then  pass  into  the  cylin- 
der as  one  large  port  at  each  end. 

The  action  of  the  valve  is  as  follows:  Steam  passes  through  the  throttle 
valve,  shown  in  Fig.  39,  to  the  interior  of  the  valve,  is  admitted  to  and 
cut  off  from  the  cylinder  by  the  inside  edges,  and  is  exhausted  from  the 
cylinder  by  the  outside  edges.  Thus  the  valve  has  steam  at  exhaust 
pressure  only,  surrounding  it.  The  lower  part  is  pressed  to  its  seat 


SINGLE-VALVE   ENGINES 


35 


and  the  upper  part  to  its  seat  by  the  low-pressure  exhaust  steam  acting 
on  the  area  of  that  part  of  the  valve  outside  of  the*  hub. 

From  the  construction,  the  two  halves  of  the  valve  are  able  to  lift 
from  their  seats  and  telescope  into  each  other,  should  there  be  excessive 
pressure  in  the  cylinder,  due  to  water,  or  other  cause. 

32.  Ported  Valves.  The  proportions  of  an  engine  are  sometimes 
such  that  it  is  difficult  or  even  impossible  to  give  sufficient  travel  to  a 
plain  slide  valve  to  open  the  port  far  enough  to  give  proper  admission 
of  steam.  Furthermore,  a  plain  slide  valve  gives  a  very  narrow  port 
opening  at  first  and  requires  some  time  to  open  the  port  enough  to  allow 
free  flow  of  steam.  Consequently  the  piston  may  have  moved  some 
distance  from  the  end  of  the  stroke  before  full  steam  pressure  is  attained 
in  the  cylinder.  Similarly  when  the  valve  is  covering  the  port,  the  clos- 
ing is  gradual,  with  a  consequent  dropping  of  pressure.  For  these  and 


FIG.  40.     Double-ported  Valve,  Marine  Type. 

other  similar  reasons,  valves  are  often  constructed  with  auxiliary  pas- 
sages through  them. 

Fig.  40  is  a  section  of  a  valve,  known  as  a  double-ported  valve,  used 
on  marine  engines.  The  two  openings  marked  " steam"  pass  completely 
through  the  valve,  and  thus  the  live  steam  which  surrounds  the  valve 
is  enabled  to  fill  these  spaces.  At  each  end  of  the  valve  seat  are  two 
small  ports,  which  merge  into  large  ones  connecting  with  the  cylinder. 
Each  of  these  ports  has  a  valve  foot  covering  it,  the  feet  on  the  same 
end  being  duplicates.  It  is  apparent  that  both  ports  on  the  same  end 
are  uncovered  simultaneously  and  the  fact  that  there  are  two  openings 
does  much  to  overcome  the  difficulties  above  mentioned. 

Figs.  41  and  42  are  sections  through  a  ported  valve  which  has  some- 
times been  called  a  "  Trick  valve. "  It  differs  from  the  ordinary  slide 


36  MECHANISM  OF   STEAM  ENGINES 

valve  by  having  the  passage  A  cored  through  it.  Referring  to  Fig.  41, 
where  the  valve  is  in  mid-position,  the  distance  from  the  edge  K  to  the 
edge  M  of  the  recess  in  the  seat  is  just  equal  to  the  head-end  steam  lap, 
so  that  when  the  steam  begins  to  flow  into  the  head-end  port  past  the 
head-end  edge  of  the  valve  in  the  usual  way  it  also  begins  to  flow  past 


FIG.  41.     Allen  Locomotive  Valve  in  Mid-position. 

the  edge  K  into  the  passage  A  and  around  into  the  head-end  port.  In 
Fig.  42  the  valve  is  shown  with  the  port  open  a  little  and  the  action  of 
the  auxiliary  passage  A  is  evident.  When  the  valve  has  moved  farther 
toward  the  crank  end  the  passage  A  is  stopped  off  by  the  head-end  bridge, 
but  this  is  not  important  as  the  main  opening  in  the  head-end  port  will 


FIG.  42.     Allen  Locomotive  Valve  at  Lead  Opening. 

then  be  ample.  When  the  valve  moves  back  and  approaches  head-end 
cut-off  the  auxiliary  passage  again  comes  into  action  and  the  flow  through 
this  passage  is  cut  off  at  the  same  time  that  the  regular  cut-off  takes 
place.  The  action  is  exactly  similar  when  steam  is  flowing  into  the 
crank  end.  The  effect  of  the  auxiliary  passage  is  to  produce  the  equiv- 


SINGLE-VALVE   ENGINES 


37 


alent  of  a  more  rapid  opening  of  the  port  at  admission  and  a  more  rapid 
closing  at  cut-off.  This  valve  has  been  used  to  a^  considerable  extent 
on  locomotives. 

Figs.  43  and  44  show  the  valve,  valve  chest  and  cylinder  of  a  Ridg- 


FIG.  43.     Valve,  Valve  Chest  and  Pressure  Plate  of  Ridgway  Engine. 

way  engine.     This  valve  has  the  " trick"  feature  for  exhaust  as  well  as 
for  admission.     The  valve  V  is  a  rectangular  frame  sliding  between  its 


FIG.  44.     Cylinder,  Valve  Chest  and  Valve  of  Ridgway  Engine. 

seat  and  the  pressure  plate  P,  which  encloses  it  on  three  sides.  The 
pressure  plate  has  auxiliary  passages  through  it  which  bend  around  and 
communicate  with  the  cylinder  ports.  The  arrows  in  Fig.  44  show  the 


38  MECHANISM  OF   STEAM  ENGINES 

steam  flowing  into  the  head  end  and  out  of  the  crank  end  in  the  usual 
way  and  also  through  the  auxiliary  passages. 

33.  Crank  and  Eccentric  for  Piston  Valve.  Piston  valves  are  usu- 
ally designed  to  have  the  supply  of  high  steam  around  the  middle  por- 
tion of  the  valve,  with  the  exhaust  passing  out  at  the  ends.  That  is, 
they  "take  steam  at  the  middle."  The  inside  laps  Lh  and  LC)  Fig.  33, 
are,  therefore,  the  steam  laps,  and  the  outside  laps  N^  and  Nc  are  the 
exhaust  laps.  This,  of  course,  necessitates  that  the  displacement  and 
direction  of  motion  of  the  valve  for  any  given  event  must  be  just  the 


FIG.  45.    D  Valve  Direct.  FIG.  4$a.    Piston  Valve  Direct. 


FIG.  46.     D  Valve  with  Reversing  FIG.  46a.    Piston  Valve  with  Reversing 

Rocker.  Rocker. 

reverse  of  that  for  a  plain  slide  valve  taking  steam  on  the  outside.  This 
reversal  of  direction  is  accomplished  by  setting  the  eccentric  diamet- 
rically opposite  on  the  shaft. 

Fig.  45  indicates  the  position  of  the  eccentric  relative  to  the  crank  for 
the  ordinary  D  valve  driven  direct;  Fig.  45 a  for  a  piston  valve  which 
takes  steam  in  the  middle,  driven  direct;  Fig.  46  for  a  D  valve  driven 
through  a  reversing  rocker;  Fig.  46a  for  a  piston  valve  driven  through  a 
reversing  rocker.  Comparing  Figs.  45  and  46a  it  will  be  seen  that  the 
eccentric  is  set  the  same  for  a  D  valve  driven  direct  and  a  piston  valve 
driven  through  a  reversing  rocker.  A  similar  comparison  between 
Figs.  45  a  and  46  shows  the  same  setting  for  a  D  valve  driven  through  a 
rocker  as  for  a  piston  valve  direct. 


CHAPTER  III 
VALVE  DIAGRAMS 

34.  In  order  to  design  a  valve  and  its  driving  mechanism  to  accomplish 
certain  ends  or  to  investigate  the  action  of  a  given  valve  gear  it  is  desir- 
able to  have  some  graphical  method  whereby  the  displacement  of  the 
valve  may  be  quickly  determined  for  any  known  piston  or  crank  posi- 
tion or  vice  versa.  There  are  a  number  of  such  diagrams  which  have 
been  used.  The  most  common  of  these  are  as  follows: 

Valve  ellipse  —  applicable  to  any  valve. 


Zeuner's  diagram .... 
Reuleaux  diagram. . .  . 
Bilgram  diagram 


Applicable  only  to  valves  having  har- 
monic or  approximately  harmonic  mo- 
tion. 


All  of  these  diagrams  are  satisfactory  and  we  shall  study  the  valve 
movements  by  the  aid  of  one  of  these,  except  in  special  cases  where 
other  methods  may  be  more  convenient.  Any  'such  diagram  must  be 
interpreted  by  intelligent  reference  to  the  actual  mechanism  to  which  it 
is  being  applied.  The  diagram  will  mean  nothing  to  the  person  who 
merely  memorizes  certain  facts  about  it  and  who  tries  to  read  results 
from  it  in  the  light  of  what  he  remembers.  He  must  be  able,  when 
looking  at  the  diagram,  to  picture  in  his  mind  just  what  the  valve,  valve 
gear,  piston  and  crank  are  doing  at  any  given  time.  Unless  this  con- 
dition is  fulfilled  the  use  of  any  diagram  is  liable  to  lead  to  serious  errors. 
35.  The  Valve  Ellipse.  The  valve  ellipse  is  a  curve  plotted  with 
piston  positions  for  abscissae  and  valve  displacements,  measured  from 
mid-position,  corresponding  to  those  piston  displacements,  for  ordinates. 
It  is  customary  to  plot  piston  displacements  to  a  reduced  scale  and  valve 
displacements  full  size.  Displacements  of  the  valve  toward  the  crank 
end  are  usually  plotted  above  the  datum  line  and  displacements  toward 
the  head  end  below  the  line.  This  point  is  not  essential  however.  If 
the  motions  of  both  valve  and  piston  are  harmonic,  the  curve  is  a  true 
ellipse,  but  if  the  motion  of  either  valve  or  piston  is  not  harmonic,  then 
the  curve  deviates  from  the  true  ellipse.  The  name  valve  ellipse  is 

39 


40  MECHANISM  OF  STEAM  ENGINES 

applied,  whatever  the  form  of  the  curve.     An  engine  may  be  made  to 
draw  its  own  ellipse  by  means  of  a  simple  attachment. 

Fig.  47  is  a  valve  ellipse  for  an  engine  with  a  plain  D  valve,  having 
approximately  harmonic  motion.  The  piston  motion  is  not  harmonic 
and  the  distortion  of  the  ellipse  due  to  this  fact  is  apparent.  The  piston 
stroke  is  plotted  at  a  small  scale,  while  the  valve  displacements  may  be 
assumed  to  be  full  size.  Ordinates  above  XhXc  indicate  valve  displace- 
ments towards  the  crank  end.  Let  us  start  with  the  piston  at  A  moving 
toward  the  head  end.  The  ellipse  crosses  the  line  XhXc  at  this  point 
and  therefore  the  valve  is  in  mid-position  and  moving  toward  the  crank 
end.  When  the  piston  reaches  the  end  of  the  stroke  the  valve  is  dis- 


FIG.  47. 

placed  a  distance  W.  The  piston  now  starts  on  its  forward  stroke,  the 
valve  still  continuing  to  move  toward  the  crank  end  until  the  piston 
reaches  0  when  the  valve  attains  its  greatest  displacement  and  starts 
back  toward  the  head  end.  When  the  piston  is  at  G  the  valve  is  again 
in  mid-position,  moving  toward  the  head  end.  When  the  piston  reaches 
P  on  the  return  stroke  the  valve  has  its  greatest  displacement  toward 
the  head  end. 

The  application  of  the  diagram  may  be  seen  from  the  following:  If 
a  line  is  drawn  parallel  to  and  above  XhXCJ  at  a  distance  from  it  equal 
to  the  head-end  steam  lap,  this  line  cuts  the  ellipse  at  B  and  E.  There- 
fore head-end  admission  occurs  when  the  piston  is  at  A  h  since  at  that 


VALVE   DIAGRAMS  41 

time  the  valve  is  displaced  toward  the  crank  end  an  amount  equal  to 
the  head-end  steam  lap  and  is  moving  toward  the,  crank  end.  At  the 
time  the  piston  is  at  the  head-end  dead  point  (so  close  to  Ah  that  the 
difference  barely  shows  in  the  drawing)  the  valve  displacement  is  W. 
Therefore  the  head-end  lead  is  W  minus  AhB.  By  dropping  a  perpen- 
dicular from  E  meeting  XhXc  at  Ch  the  piston  position  for  head-end 
cut-off  is  found  since  at  that  time  the  valve  has  the  same  displacement 
as  at  head-end  admission  and  is  moving  toward  the  head  end  which  is 
the  proper  position  and  direction  for  a  D  valve  to  close  the  head-end 
steam  port. 

Since  a  D  valve  with  a  head-end  exhaust  lap  must  be  displaced  to- 
ward the  head  end  in  order  to  bring  the  inside  edge  of  the  valve  to  the 
edge  of  the  port,  if  we  draw  a  line  parallel  to  XhXc,  below  it  and  distant 
from  it  equal  to  the  head-end  exhaust  lap,  then  release  and  compression 
for  the  head  end  will  occur  when  the  piston  is  at  Rh  and  Kh  respectively. 
The  events  of  the  stroke,  lead  opening,  etc.,  for  the  crank  end  may  be 
found  in  a  similar  manner.  Of  course  the  process  may  be  reversed; 
for  example,  if  the  per  cent  of  stroke  at  which  head-end  cut-off  is  desired 
is  known,  the  head-end  steam  lap,  lead  opening,  etc.,  may  be  found. 
As  every  irregularity  in  the  motion  of  both  piston  and  valve  are  taken 
into  account  in  drawing  the  valve  ellipse,  it  is  a  particularly  useful  dia- 
gram when  the  motion  of  the  valve  deviates  greatly  from  harmonic,  as 
it  does  in  some  complicated  valve  gears. 

36.  Zeuner's  Diagram.  If,  instead  of  plotting  the  valve  displace- 
ments as  ordinates  in  a  rectangular  plot  as  in  the  valve  ellipse,  they  are 
plotted  on  the  center  line  of  the  crank,  we  obtain  a  polar  curve  whose 
angles  are  crank  angles  and  whose  distances  from  the  pole  are  valve 
displacements.  Therefore  the  valve  displacement  for  any  given  crank 
position  as  OB,  Fig.  48,  is  the  distance  OT  from  the  origin  (center  of  the 
crank  shaft)  to  the  point  where  the  center  line  of  the  crank  cuts  the 
curve.  For  the  position  OM  and  OMi  where  the  crank  line  is  tangent 
to  the  curve  the  valve  is  in  mid-position.  When  the  crank  line  inter- 
sects the  upper  curve  the  valve  is  displaced  to  one  side  of  mid-position 
and  when  it  intersects  the  lower  curve  the  displacement  of  the  valve  is 
toward  the  other  side  of  mid-position.  With  the  ordinary  slide  valve 
driven  direct  the  upper  curve  represents  displacements  toward  the 
crank  end  and  the  lower  curve  toward  the  head  end. 

If  this  curve  be  plotted  for  a  valve  which  has  harmonic  motion  the 
two  curves  become  circles,  as  shown  in  Fig.  49,  the  diameter  of  each 


42  MECHANISM  OF  STEAM  ENGINES 

being  equal  to  one  half  the  valve  travel.  This  can  readily  be  seen  by 
actually  plotting  such  a  curve,  assuming  the  valve  to  have  harmonic 
motion.  The  following  is  a  proof  that  the  curves  are  circles: 


FIG.  48. 


In  Fig.  50  let  OB  be  any  crank  position.  If  the  angle  BON  is  the 
angle  between  the  crank  and  eccentric  and  the  eccentricity  is  ON  the 
center  of  the  eccentric  is  at  N.  Then  the  valve  displacement  for  har- 


Eccentrjc 


monic  motion  is  OD.  If  now,  instead  of  drawing  from  N  a  perpendicular 
to  XhXc  to  find  OD,  we  assume  the  eccentric  to  be  swung  back  to  coin- 
cide with  the  crank  and  swing  the  line  to  which  we  draw  the  perpendicu- 
lar back  through  the  same  angle  we  get  line  OE.  The  distance  OT\9 


VALVE  DIAGRAMS  43 

found  by  drawing  a  perpendicular  from  B  to  OE,  will  give  us  the  same 
value  as  OD.  That  is,  OT\  is  the  valve  displacement  when  the  crank  is 
at  OB,  the  length  OB  being  made  equal  to  one  half  the  valve  travel.  The 
same  result  is  evidently  obtained  by  drawing  from  E  a  perpendicular  to 
the  crank.  We  then  have  one  point  T  on  the  polar  plot  shown  in  Fig. 
49.  If  now  we  can  show  that  the  locus  of  the  point  T  is  a  circle,  as  the 
crank  revolves,  we  will  have  proved  that  the  curves  in  Fig.  49  are  circles. 
To  show  that  the  locus  of  T  is  a  circle  draw  a  line  from  T  to  P,  the 
middle  of  OE.  Draw  PW  perpendicular  to  ET.  Then  from  the  simi- 
larity of  the  triangles  EPW  and  EOT,  EW  must  be  equal  to  WT  since 
EP  =  PO.  Then  in  the  two  right  triangles  EWP  and  TWP,  EW  =  TW 
and  WP  is  common.  Therefore  the  triangles  are  equal  and  TP  =  EP  = 
OP.  Therefore  the  points  E,  T  and  0  are  on  a  circle  whose  center  is 
P.  Since  0  and  E  are  fixed  points  and  OB  is  any  crank  position,  the 
point  T  will  always  be  on  the  circle.  A  similar  line  of  reasoning  of  course 
applies  to  the  lower  curve.  The  two  circles  are  called  valve  circles. 

In  Fig.  49  the  lines  OM  and  OMi,  which  are  tangent  to  the  circles  at 
0,  show  crank  positions  when  the  valve  is  in  mid-position.  Therefore 
angle  XhOMi  and  XCOM  are  equal  to  the  angular  advance.  These  lines 
are,  of  course,  perpendicular  to  E\OE,  therefore  the  angle  EOY  is  equal 
to  the  angular  advance. 

Following  the  motion  of  the  valve  from  Fig.  49,  —  when  the  crank 
is  at  OEi  the  valve  has  its  extreme  displacement  toward  the  head  end 
and  is  just  starting  to  move  toward  the  crank  end;  it  reaches  mid- 
position  when  the  crank  reaches  OM\\  after  the  crank  passes  OM\  the 
valve  is  displaced  towards  the  crank  end  and  continues  to  move  toward 
the  crank  end  until  the  crank  reaches  OE.  Then  the  valve  has  its  ex- 
treme displacement  toward  the  crank  end  and  starts  to  move  toward 
the  head  end.  It  reaches  mid-position  again  when  the  crank  gets  to 
OM ,  and  again  goes  to  the  head-end  side  of  mid-position  after  the  crank 
passes  OM. 

37.  Application  of  the  Zeuner's  Diagram.  The  method  of  using  the 
Zeuner's  diagram  is  similar  to  that  for  the  valve  ellipse  with  the  modi- 
fications necessary  because  it  is  a  polar  plot.  Referring  to  Fig.  51, 
arcs  are  swung  about  the  pole  0  with  radii  equal  to  the  respective 
laps  and  on  the  side  of  the  mid-position  line  MOM\  toward  which  the 
valve  must  be  displaced  to  have  the  various  edges  act.  The  crank 
positions  for  the  various  events  are  found  by  drawing  from  0  through 
the  points  where  the  lap  circles  cut  the  valve  circles,  identifying  the 


44 


MECHANISM  OF  STEAM  ENGINES 


various  events  by  thinking  which  way  the  valve  is  displaced  and  which 
way  it  is  moving. 

C.E.  Exhaust  Lap 
E 

E.  Steam  Lap 
H.E.  Lead 

H.E.  Lead 

Angle 


C.E.  Steam  Lap 


FIG.  51. 


38.  Reuleaux  Diagram.  A  diagram  which  in  some  cases  is  more 
convenient  than  the  Zeuner's  diagram  is  the  one  sometimes  known  as 
the  Reuleaux  diagram. 

In  Fig.  52  a  circle  is  drawn  with  0  as  a  center  and  radius  equal  to  one 
half  the  valve  travel.  We  will  refer  to  this  circle  as  the  eccentric  circle 
since  it  is  the  path  of  the  center  of  the  eccentric  if  the  valve  is  direct 
connected,  or  if  the  valve  is  driven  by  an  unequal  armed  rocker  it  would 
be  the  path  of  the  center  of  a  direct-connected  eccentric  which  would 
give  the  same  valve  travel.  Let  OC  be  the  crank  and  let  the  angle 
between  the  crank  and  the  eccentric  be  K.  For  any  crank  position  OC 
the  eccentric  center  is  at  N,  found  by  laying  off  the  angle  K  ahead  of 
or  behind  the  crank  according  as  the  eccentric  is  set  to  lead  or  follow  the 
crank.  If  the  valve  has  harmonic  motion  its  displacement  is  OD.  If, 
now,  we  draw  the  line  EiOE  making  the  angle  XCOE  equal  to  K,  and 
if  from  B,  where  the  crank  line  crosses  the  eccentric  circle,  we  drop  a 
perpendicular  BT  to  OE,  then  OT  is  equal  to  OD  and  is  the  valve  dis- 


VALVE   DIAGRAMS 


45 


placement  for  this  crank  position.     The  same  holds  true  for  any  crank 
position.  0 

The  construction  for  this  diagram  merely  consists,  therefore,  in  draw- 
ing the  eccentric  circle,  and  then  the  line  E\OE  making  an  angle  XCOE 


Ec.c.entrrc 


Crank 


G 


Eccentric 


FIG.  52. 


FIG.  53. 


with  the  engine  center  line  equal  to  the  angle  which  the  eccentric  makes 
with  the  crank.  Then  the  valve  displacement  for  any  given  crank 
position  is  the  distance  from  the  center  of  the  circle  to  the  foot  of  a  per- 
pendicular drawn  from  the  point  where  the  crank  cuts  the  eccentric 
circle  to  the  line  EiOE.  The  line  EiOE  may  be  called  the  reference  line. 

The  construction  for  a  case  where  the  eccentric  follows  the  crank  is 
shown  in  Fig.  53,  where  the  letters  have  the  same  meaning  as  in  Fig.  52. 

It  will  be  noticed  that  the  reference  line  of  the  Reuleaux  diagram  is 
the  same  line  as  the  valve  circle  diameter  of  the  Zeuner's  diagram. 

39.  Application  of  the  Reuleaux  Diagram.     Fig.   54  is  a  complete 
diagram  which  is  self  explanatory. 

40.  The  Bilgram  Diagram.    With  center  0,  Fig.  55,  draw  a  circle 
whose  radius  is  one  half  the  valve  travel.     Let  OM  be  any  position  of 
the  center  line  of  the  crank,  making  an  angle  6  with  the  head-end  dead- 
point  position.     From  OXC  lay  up  the  angle  XCOB  equal  to  the  angular 
advance,  getting  the  point  B.     From  B  drop  the  perpendicular  BD  to 
the  center  line  of  the  crank  or  of  the  crank  produced.     Then  BD  is  the 
valve  displacement  for  the  crank  position  OM. 

Proof:   In  Fig.  56,  if  SC  is  the  center  line  of  the  crank  corresponding 


46 


MECHANISM   OF   STEAM   ENGINES 


.E.  Exhaust  Lap 

E.  Steam  Lap 


G.E.  Steam.  Lap 
H.E.  Exhaust  Lap 


FIG.  54. 


FIG.  55. 


VALVE  DIAGRAMS  47 

to  position  OM  in  Fig.  55,  E  is  the  actual  position  of  the  eccentric  center. 
Then  ST  is  the  valve  displacement  if  the  valve  has  harmonic  motion. 

EST  =  180°  -  (90°  +  a  +  0) 

=     90°  -  (a  +  0). 
In  Fig.  55, 

BOD  =  (a  +  0). 

Therefore  OBD  =  90°  -  (a  +  0). 

Hence  OBD  =  EST  and  since  OB  =  SE  the  triangles  EST 

and  OBD  are  equal. 
Therefore  BD  =  57. 

Referring  still  to  Fig.  55,  if  L  is  equal  to  the  head-end  steam  lap  and 
N  the  head-end  exhaust  clearance, 

A  h  is  the  crank  position  for  H.E.  admission. 
Ch  is  the  crank  position  for  H.E.  cut-off. 
Rh  is  the  crank  position  for  H.E.  release. 
Kh  is  the  crank  position  for  H.E.  compression. 


CHAPTER  IV 
TYPICAL  PROBLEMS   ON  THE   SLIDE-VALVE   ENGINE 

41.  The  following  examples  will  serve  to  illustrate  the  manner  of 
studying  the  action  or  design  of  a  slide-valve  mechanism,  certain  pro- 
portions of  which  are  known.     An  understanding  of  the  constructions 
here  given  will  also  give  more  familiarity  with  the  slide  valve,  its  possi- 
bilities and  its  limitations,  than  could  be  obtained  in  any  other  way. 
The  student  should  constantly  guard  against  becoming  so  involved  in 
the  geometry  of  the  diagrams  that  he  loses  sight  of  the  real  mechanism. 

42.  Given:  Ratio  connecting  rod  to  crank         =  4  to  i. 

Ratio  eccentric  rod  to  eccentricity     =  6  to  i. 

Valve  to  be  a  plain  D  valve  driven  direct.  Angle  between  crank  and  eccen- 
tric known;  eccentricity  known;  piston  positions  for  cut-off  and  compres- 
sion known  j or  both  ends. 

To  find:  Laps,  leads  and  piston  positions  for  release. 

Here  the  ratio  of  eccentric  rod  to  eccentricity  is  so  small  that  the 
valve  motion  departs  materially  from  harmonic  motion.  Consequently 
any  one  of  the  diagrams  which  depends  upon  harmonic  motion  of  the 
valve  would  be  inaccurate  in  this  case.  The  valve  ellipse,  however,  takes 
into  account  all  the  irregularities  and  can  be  used.  The  first  step  will 
be  the  plotting  of  the  ellipse.  This  is  shown  in  Fig.  58,  where  the  valve 
displacements  are  taken  from  Fig.  57.  The  method  of  plotting  the 
ellipse  should  be  clear  from  the  discussion  in  §  35.  Having  plotted  the 
ellipse,  Fig.  58,  the  piston  positions  for  cut-off  and  compression  are 
located  on  the  stroke  of  the  cross  head  pin.  These  are  lettered  Ch)  Cc, 
Kh  and  Kc.  From  these  positions  perpendiculars  are  erected  and  the 
lap  lines  drawn  through  the  points  where  these  perpendiculars  cut  the 
ellipse.  To  determine  which  intersection  to  use  in  each  case  it  is  neces- 
sary to  reason  out  which  way  the  piston  is  moving  and  which  way  the 
valve  is  displaced  and  moving  for  the  event  under  consideration.  The 
lead  is  found  by  taking  the  difference  between  the  laps  and  the  displace- 
ment of  the  valve  when  the  crank  is  on  the  dead  point.  The  piston 

48 


TYPICAL  PROBLEMS  ON  THE   SLIDE-VALVE  ENGINE 


49 


50  MECHANISM   OF   STEAM   ENGINES 

positions  for  release  are  found  by  dropping  perpendiculars  from  the  proper 
intersections  of  the  exhaust  lap  lines  with  the  ellipse. 

43.  Given:  Ratio  of  connecting  rod  to  crank.  Valve  to  be  a  plain  D 
valve  to  give  equal  leads  of  known  amount.  Valve  motion  practically  har- 
monic. Head-end  steam  lap  known,  valve  travel  known,  exhaust  laps  equal 
and  known. 

To  find:  Crank-end  steam  lap,  and  per  cent  of  stroke  at  which  cut-off, 
release  and  compression  occur. 


FIG.  59. 

Since  the  valve  motion  may  be  assumed  harmonic,  either  Zeuner's, 
Reuleaux  or  Bilgram  diagrams  may  be  used.  We  will  work  by  both 
the  Zeuner's  and  Reuleaux  diagrams.  Referring  to  Fig.  59  draw  the 
line  XcXh  as  the  center  line  of  the  engine,  and,  choosing  point  O  as  the 
center  of  the  crank  shaft,  draw  the  crank-pin  circle  at  any  convenient 


TYPICAL  PROBLEMS   ON  THE   SLIDE-VALVE   ENGINE  51 

scale.  In  this  figure  as  in  the  succeeding  ones  the  stroke  of  the  crosshead 
pin  is  reproduced  directly  under  the  crank-pin  circle  to  save  space, 
although  in  actually  making  the  drawing  it  was  drawn  in  its  proper  posi- 
tion on  the  line  XcXh  produced  as  shown  in  Fig.  3.  About  O  as  a  center 
draw  a  circle  with  radius  equal  to  the  head-end  steam  lap.  From  the 
point  m  where  this  cuts  XcXh  lay  off  mn  equal  to  the  given  head-end  lead. 
From  n  erect  a  perpendicular  to  XcXh  and  from  0  with  a  radius  equal  to 
one  half  the  valve  travel  cut  this  perpendicular  at  E.  Then  OR  is  the 
diameter  of  the  upper  valve  circle  for  the  Zeuner's  diagram.  Produce 
EO  to  Eij  making  OEi  =  OE.  On  these  two  lines  draw  the  valve  circles. 
The  lines  OAh  and  OCh  drawn  through  the  intersections  of  the  head-end 
steam-lap  circle  with  the  upper  valve  circle  show  the  crank  positions  for 
head-end  admission  and  cut-off  respectively.  Since  the  crank-end  lead 
was  given  as  equal  to  the  head-end  lead  the  crank-end  steam  lap  must 
be  equal  to  the  head-end  steam  lap.  This  is  evident  from  the  geometry 
of  the  figure.  About  0  draw  a  circle  with  radius  equal  to  the  given 
exhaust  laps  (which  were  assumed  to  be  equal).  When  the  crank  is  at 
OKh,  drawn  through  the  intersection  of  the  exhaust-lap  circle  with  the 
lower  valve  circle,  the  valve  is  displaced  toward  the  head  end  an  amount 
equal  to  the  head-end  exhaust  lap,  and  is  moving  toward  the  crank  end. 
Therefore  head-end  compression  is  beginning  at  OKh.  When  the  crank 
reaches  ORC  the  valve  is  displaced  toward  the  crank  end  an  amount 
equal  to  the  crank-end  exhaust  lap  and  is  moving  toward  the  crank  end. 
Therefore  crank-end  release  is  occurring.  Similarly,  when  the  crank  is 
at  OKC  the  valve  is  displaced  toward  the  crank  end  an  amount  equal  to 
the  crank-end  exhaust  lap  and  is  moving  toward  the  head  end,  therefore 
crank-end  compression  is  beginning;  and  a,t»ORh  the  valve  is  displaced 
toward  the  head  end  an  amount  equal  to  the  head-end  exhaust  lap  and 
is  moving  toward  the  head  end  so  that  head-end  release  is  occurring. 
The  crosshead  positions  corresponding  to  these  several  crank  positions 
are  found  in  percentages  of  the  stroke  as  described  in  §  6.  These  are 
shown  on  the  stroke  line  in  Fig.  59. 

Fig.  60  is  the  solution  of  the  same  problem  by  means  of  the  Reuleaux 
diagram.  The  crank-pin  circle  is  drawn  as  above  described.  About 
0  is  drawn  the  eccentric  circle  with  radius  equal  to  one  half  the  valve 
travel.  From  O  with  radius  Om  equal  to  head-end  steam  lap  plus  head- 
end lead  an  arc  is  drawn,  and  through  /,  where  the  eccentric  circle  cuts 
XcXh,  a  line  is  drawn  tangent  to  this  arc.  EOEi  drawn  perpendicular  to 
this  line  is  the  reference  line  of  the  Reuleaux  diagram.  Lay  off  Op  equal 


52 


MECHANISM  OF  STEAM  ENGINES 


to  the  head-end  steam  lap.  Then  a  line  through  p  perpendicular  to 
will  intersect  the  eccentric  circle  at  V  and  W,  and  OAh  and  OCh  drawn 
through  V  and  W  respectively  give  the  crank  position  for  head-end 
admission  and  cut-off.  OZ  and  OS  being  made  equal  to  the  head-end 


Crank  Ctrcle 


FIG.  60. 

and  crank-end  exhaust  laps  respectively  give  the  points  through  which 
to  draw  perpendiculars  to  EOEi  to  find  the  crank  positions  for  release 
and  compression. 

An  inspection  of  the  stroke  lines  in  Figs.  59  and  60  indicates  that  the 
cut-offs  are  unequal,  showing  the  truth  of  statement  i  under  §  28. 

44.  Given:  Ratio  of  connecting  rod  to  crank.  D  valve  with  harmonic 
motion.  Valve  travel  known;  cut-offs  equal  and  percentage  known;  crank- 
end  lead  known;  compressions  equal  and  percentage  known. 

To  find:  Steam  and  exhaust  laps,  head-end  lead,  percentage  stroke  for 
releases. 


TYPICAL  PROBLEMS   OF  THE   SLIDE-VALVE   ENGINE 


53 


Referring  to  Fig.  61,  the  crank  circle  is  drawn  and  the  stroke  line  laid 
off  as  before;  the  eccentric  circle  is  also  drawn  witji  radius  equal  to  half 
the  valve  travel.  With  center  t  where  the  eccentric  circle  cuts  the  crank 
end  of  XcXh  draw  a  circle  with  radius  equal  to  the  crank-end  lead.  Find 
crank  positions  OCC  for  crank-end  cut-off  and  OCn  for  head-end  cut-off. 
From  M  where  OCC  cuts  the  eccentric  circle  draw  a  line  tangent  to  the 


FIG.  61. 

lead  circle  cutting  the  eccentric  circle  at  N.  Then  ON  will  give  the  crank 
positions  for  crank-end  admission  and  EiOE  perpendicular  to  MN  will 
be  the  valve  circle  diameters.  Od  will  be  the  crank-end  steam  lap.  The 
correctness  of  this  construction  can  be  understood  by  thinking  of  its  simi- 
larity to  the  Reuleaux  diagram.  The  valve  circles  are  next  drawn  and 
from  OCh  the  head-end  steam  lap  and  therefore  head-end  lead  are  found. 
The  crank  positions  for  the  equal  compressions  may  now  be  constructed, 


54 


MECHANISM   OF  STEAM  ENGINES 


and  from  these  the  exhaust  laps  and  the  crank  positions  for  release 
determined.  Attention  is  called  here  to  the  method  of  finding  the  exact 
point  of  intersection  of  a  crank  line,  as  for  example  OKh,  with  the  valve 
circle,  by  dropping  a  perpendicular  from  E  to  OKh.  Since  both  com- 


Eccentric  Circle 


FIG.  62. 

pressions  (see  OKh  and  OKC)  occur  after  the  valve  has  passed  mid-posi- 
tion the  exhaust  laps  OZ  and  05  must  be  negative  (see  §  28,  statement  5). 

Fig.  62  is  the  solution  of  the  preceding  problem  by  means  of  the  Reu- 
leaux  diagram. 

From  Figs.  61  and  62  the  great  inequality  of  steam  laps  resulting  from 
making  the  cut-offs  equal  is  apparent,  with  the  consequent  inequality 
of  leads.  This  bears  out  statement  2,  §  28.  The  inequality  of  the 
exhaust  laps  or  clearances  due  to  equal  compression  is  also  evident  (see 
statement  3,  §  28). 


TYPICAL  PROBLEMS  OF  THE   SLIDE-VALVE  ENGINE 


55 


45.  Fig.  63  is  the  solution  by  means  of  Zeuner's  diagram  and  Fig.  64 
is  the  solution  by  the  Reuleaux  diagram  of  a  problem  in  which  the  data 
is  the  same  as  in  §  44  except  that  the  leads  are  made  equal  and  both 
exhaust  laps  are  taken  as  zero.  This  brings  out  no  new  point  but  shows 
again  the  effect  which  equal  laps  have  on  the  equality  of  the  cut-off 


FIG.  63. 

and  compression.  It  is  also  evident  that  with  zero  exhaust  laps  all 
exhaust  events  occur  when  the  valve  is  in  mid-position. 

46.  Given:  Piston  valve  taking  steam  at  the  middle,  direct  connected; 
valve  travel,  lead  and  angle  between  crank  and  eccentric  known;  compres- 
sions equal  at  known  percentage  of  stroke.  To  find  the  steam  and  exhaust 
laps  and  the  per  cent  stroke  of  cut-ojfs  and  releases. 

Since  with  a  piston  valve  taking  steam  at  the  middle  the  eccentric 
will  be  placed  relative  to  the  crank  as  shown  in  Fig.  45a  the  angle 


MECHANISM   OF  STEAM   ENGINES 


Fig.  65,  will  be  made  equal  to  the  angle  between  crank  and  eccentric 
(see  §  36).  On  the  line  EOE\  thus  found  draw  the  two  valve  circles 
with  diameters  equal  to  one  half  the  known  travel.  In  the  figure  the 
scale  happens  to  be  such  that  the  length  of  the  stroke  line  on  the  draw- 


v     1     •,•" 

H-p* 

|       Kc 

1       ' 

Stroke 

Cc         Rc 

FIG.  64. 

1 

ing  appears  the  same  as  the  valve  travel.  This  is,  of  course,  not  actually 
the  case,  as  the  valve  dimensions  are  drawn  full  size  while  the  crank-pin 
circle  is  at  a  much  reduced  scale.  Since  we  are  dealing  with  a  piston 
valve  where  the  inside  laps  are  the  steam  laps  the  upper  valve  circle 
represents  valve  displacements  toward  the  head  end  and  the  lower, 
toward  the  crank  end.  The  distance  On  indicates  the  amount  the  valve 
is  displaced  when  the  crank  is  on  the  head-end  dead  point.  From  n 
measure  in  the  distance  nm  equal  to  the  known  head-end  lead,  then  Om 


TYPICAL   PROBLEMS   OF  THE   SLIDE-VALVE   ENGINE 


57 


is  the  head-end  steam  lap.  In  a  similar  way  measure  in  from  d  the 
distance  dh  equal  to  the  crank-end  lead  and  Oh  is  the  crank-end  steam 
lap.  Knowing  the  steam  laps,  the  crank  positions  for  admission  and 
cut-off  and  the  per  cent  stroke  for  cut-offs  can  be  found.  Since  the 


1  1                                                                                                           1 

Ko 

Cc     Re 

FIG.  65. 

percentages  at  which  the  compressions  occur  are  known  the  crank  posi- 
tions corresponding  are  found  in  the  usual  way  and  from  these  the  ex- 
haust laps  are  determined  and  then  the  releases.  In  this  case  both 
exhaust  laps  happen  to  be  positive  but  the  head-end  lap  is  much  the 
smaller,  being  nearly  zero. 

Fig.  66  is  the  Reuleaux  diagram  for  the  same  problem. 

47.  Short  Cut-off  at  Expense  of  Other  Events.  In  the  preceding 
examples  the  data  has  been  so  chosen  that  the  cut-off  was  fairly  late  in 
the  stroke.  An  engine  running  under  normal  load  gives  more  econom- 
ical results  with  a  short  cut-off.  A  single  valve  cannot  be  designed  to 
give  short  cut-off  without  sacrificing  on  release  or  compression  or  both. 


58  MECHANISM  OF  STEAM  ENGINES 

Fig.  67  is  a  Reuleaux  diagram  for  a  slide  valve,  giving  a  small  lead, 
and  equalizing  cut-off  at  ^  stroke.  In  order  to  obtain  this  cut-off  the 
eccentric  must  be  set  at  a  large  angle  ahead  of  the  crank  with  the  result 
that  release  and  compression  are  both  very  early.  The  release  might 


FIG.  66. 


be  made  later  by  decreasing  the  exhaust  clearance,  but  that  would  make 
the  compression  still  earlier. 

Fig.  68  is  an  indicator  card  which  shows  about  what  the  steam  dis- 
tribution would  be  under  the  conditions  of  Fig.  67. 

48.  Port  Calculations  and  Valve  Layout.  In  the  design  of  a  slide 
valve  for  a  certain  engine  the  laps  are  determined  by  some  method 
similar  to  the  ones  illustrated  in  the  preceding  examples.  The  total 
length  of  the  valve  must  be  determined  by  laying  out  a  longitudinal 
section  in  which  the  ports,  bridges,  etc.,  are  proportioned  and  located 
to  give  proper  action.  The  first  step  in  this  work  is  to  determine  the 
proper  width  of  port  to  supply  steam  to  and  exhaust  it  from  the  engine 


TYPICAL  PROBLEMS  OF  THE  SLIDE-VALVE  ENGINE  59 


FIG.  67. 


FIG.  68. 


6o 


MECHANISM  OF  STEAM   ENGINES 


in  question.     The  following  method  will  serve  as  a  guide  in  selecting  a 
suitable  width  of  port. 

First  decide  upon  the  average  velocity  of  the  steam  flow.     This  may 
be  taken  as  6000  feet  per  minute  for  live  steam  and  4000  feet  per  minute 


acentric  Circle 


X.I 

Rh 

\h                                                                        \              \ 

~7"~            ~l 
C,                 /?. 

Pfrnkr                                                                      >l 

FIG.  69. 


for  exhaust  steam.  These  figures  are  conservative,  much  higher  average 
velocities  being  often  allowed.  It  is  assumed  that  the  rate  of  flow 
through  the  ports  is  such  that  the  cylinder  would  be  filled  twice  in  each 
revolution  if  steam  flowed  in  during  the  full  stroke.  The  cubic  feet  of 
steam  required  per  minute  in  the  cylinder  would  be  V  =  piston  area  in 
square  feet  X  stroke  in  feet  X  revolutions  per  minute  X  2. 


TYPICAL  PROBLEMS  OF  THE   SLIDE-VALVE  ENGINE 


6l 


Then  V  -=-  average  velocity  of  live  steam  X  144  will  be  the  port  area 
in  square  inches  necessary  for  admitting  steam  and  V  -5-  average  velocity 
of  exhaust  steam  X  144  will  be  the  port  area  in  square  inches  necessary 
for  exhaust  steam. 


Section  on  A  B 
FIG.  70. 

Since  the  exhaust  steam  will  require  the  greater  area  the  port  will  be 
calculated  for  the  exhaust  and  will  then  be  ample  for  live  steam. 

The  length  of  the  port  will  usually  be  somewhat  less  than  the  diam- 
eter of  the  cylinder  bore  and  for  purposes  of  the  port  calculation  it  may 
be  assumed  to  be  three-fourths  of  the  bore  diameter.  The  width  of  the 
port  will  then  be  equal  to  the  port  area  divided  by  port  length. 

In  laying  out  the  valve  and  a  section  of  its  seat,  Fig.  70,  we  will  assume 
that  sufficient  information  is  at  hand  to  draw  the  diagram,  Fig.  69. 
From  this  the  laps  and  travel  will  be  taken  in  constructing  Fig.  70. 


<f>2  MECHANISM  OF  STEAM  ENGINES 

Draw  the  line  representing  the  surface  of  the  valve  seat  and  lay  off 
at  a  convenient  place  hj,  the  port  width  as  calculated  by  the  method 
above  explained.  It  is  better  to  start  with  the  port  which  has  the 
greater  exhaust  lap.  From  Fig.  69  this  is  seen  to  be  the  crank  end.  The 
crank-end  steam  and  exhaust  laps  are  now  laid  off  from  h  and  j  (in  this 
case  the  exhaust  lap  is  zero)  giving  the  valve  edges  a  and  b.  Next,  choose 
a  width  of  bridge  jk  which  will  be  practicable  for  strength  and  test  to 
see  that  the  edge  a  of  the  valve  never  moves  onto  the  bridge  far  enough 
to  allow  leakage  past  it  into  the  exhaust.  In  the  figure  the  edge  a  does 
not  run  onto  the  bridge  when  it  has  its  greatest  displacement,  as  shown 
by  the  dotted  line.  In  some  cases  it  might,  hence  the  necessity  for  the 
above  test.  The  width  of  the  exhaust  cavity  kn  is  found  by  making  the 
distance  mn,  between  the  inside  edge  of  the  valve  when  it  has  its  greatest 
displacement,  and  the  nearer  edge  of  the  head-end  bridge,  equal  to  or  a 
little  greater  than  the  port  width.  Make  the  head-end  trial  width  of 
the  bridge  equal  to  the  width  of  the  crank-end  bridge,  lay  off  the  width 
of  the  port  ef  and  lay  off  the  head-end  steam  and  exhaust  laps.  Now 
test  to  see  that  steam  cannot  flow  past  edge  c  into  the  exhaust.  If  it 
does  the  bridge  ne  must  be  made  thicker.  The  height  of  the  exhaust 
cavity  H  is  more  or  less  arbitrary.  It  may  be  made  a  little  greater  than 
the  width  of  the  port. 

In  order  that  the  edges  of  the  valve  may  overtravel  the  seat  and  thus 
wear  more  evenly  the  seat  is  cut  down  from  the  points  5  and  /,  the  amount 
of  overtravel  being  \  inch  or  more. 


CHAPTER  V 
GOVERNING  DEVICES  FOR   SINGLE-VALVE  ENGINES 

49.  An  engine  is  designed  to  develop  a  certain  power  and  when  develop- 
ing this  power  it  may  be  said  to  be  running  under  normal  load.  The 
load  on  any  engine  is  likely  to  vary,  however,  over  a  wide  range.  This 
variation  must  be  met  by  a  corresponding  change  in  the  steam  supply; 
that  is,  if  the  engine  has  less  than  its  normal  amount  of  work  to  do 
either  less  steam  must  be  supplied  in  a  given  time,  or  else  the  steam  must 
be  supplied  at  a  lower  pressure.  In  locomotives,  marine  engines  and 
the  like,  the  steam  supply  is  adjusted  to  the  demand  by  the  attendant. 
Stationary  engines  are  usually  expected  to  run  at  a  practically  constant 
speed  and  to  maintain  this  speed  by  automatically  regulating  their  own 
steam  supply.  The  devices  by  means  of  which  such  regulation  is  accom- 
plished are  called  governors.  It  is  our  purpose  in  the  present  chapter 
to  consider  the  principles  of  some  of  these  governing  mechanisms  and 
the  way  in  which  different  types  of  governors  affect  the  steam  distri- 
bution. 

Reference  has  already  been  made  to  the  indicator  card  as  a  means 
of  showing  how  the  steam  is  distributed  and  as  a  measure  of  the  work 
which  the  engine  is  doing.  We  shall  make  use  of  the  indicator  card  as 
a  basis  for  comparing  the  action  of  the  various  governors  discussed. 

For  purposes  of  this  discussion  we  shall  deal  principally  with  the 
head  end  of  the  cylinder,  bearing  in  mind  that  the  same  considerations 
apply  to  both  ends. 

In  Fig.  12  the  area  EBCRD  is  a  measure  of  the  work  done  by  the 
steam  during  the  forward  stroke.  Similarly  the  cross-hatched  area 
represents  the  work  done  on  the  steam  during  the  return  stroke.  The 
difference  of  these  areas,  that  is,  the  area  enclosed  by  the  card  ABCRK, 
represents  the  net  work  done  on  the  piston  by  the  steam  per  revolution 
in  one  end  of  the  cylinder.  Evidently,  if  the  speed  of  the  engine  re- 
mains constant  the  power  will  be  increased  by  increasing  the  area  of  the 
card  and  decreased  by  decreasing  the  area. 

63 


64 


MECHANISM  OF  STEAM  ENGINES 


Di 


A-\r 


K 


FIG.  71. 


FIG.  72. 


o 


K 


FIG.  73. 


GOVERNING   DEVICES  FOR   SINGLE-VALVE  ENGINES 


The  purpose  of  the  various  governing  devices  is  to  change  the  area  of 
the  card  as  the  load  on  the  engine  varies,  the  change  of  area  being  such 
that  the  speed  will  remain  practically  constant. 

Fig.  71  is  a  card  representing  the  conditions  when  the  engine  is  carrying 
a  full  load,  the  cut-off  being  very  late.  If  now,  the  load  is  partly  removed 
from  the  engine  so  that  the  work  done  on  the  head  end,  instead  of  re- 
quiring a  card  as  in  Fig.  71,  requires  a  card  of  less  area,  this  area  may 
be  reduced  in  one  of  two  ways. 

50.  Throttling  Governors.     One  of  these  ways  is  by  "  throttling  " 
the  steam  supply,  keeping  the  cut-off  the  same.     Fig.  72  shows  the  card 
for  such  a   condition   and  Fig.   74 

shows  a  throttling  governor  such  as 
might  be  used  to  accomplish  the  re- 
sult. The  operation  of  the  governor 
is  as  follows:  The  pulley  K  is 
belted  to  the  engine  shaft,  and,  by 
means  of  the  shaft  S,  sleeve  T  and 
the  connecting  bevel  gears,  causes 
the  balls  B  to  revolve  about  a  ver- 
tical axis.  Centrifugal  force  causes 
the  balls  to  fly  out  and  the  levers 
L  force  the  shaft  M  downward  par- 
tially closing  the  valve  A,  through 
which  the  steam  must  pass  to  reach 
the  steam  chest. 

51.  Flywheel     Governors.      The 
second  method  for  reducing  the  area 
is  by  causing  the  valve  to  cut-off 
earlier,  as  represented  in   Fig.   73, 
where  the  area  is  practically  the  same 
as  in  Fig.  72,  but  the  pressure  of  the 
steam  during  admission  is  the  same 

as  in  Fig.  71.  While  it  is  not  possible  at  this  time  to  go  into  a  dis- 
cussion of  the  relative  advantages  of  the  two  cards  (Figs.  72  and  73) 
attention  might  be  called  to  the  fact  that  in  Fig.  72  the  steam  begins 
to  exhaust  at  a  relatively  high  pressure,  whereas  in  Fig.  73  the  steam 
expands  down  to  a  much  lower  point  before  being  sent  out  of  the 
cylinder. 

In  making  these  cards  it  has  been  assumed  that  all  the  events  of  the 


K 


FIG.  74. 


66  MECHANISM  OF  STEAM  ENGINES 

stroke  have  remained  unchanged  except  cut-off.  With  a  single  valve, 
however,  this  would  be  impossible,  as  any  change  in  the  eccentric  which 
would  vary  the  cut-off  would  also  vary  the  lead  and  the  release  and 
the  compression.  What  follows  in  this  chapter  applies  to  single-valve 
engines  with  single  eccentric.  Later  we  shall  consider  control  of  steam 
by  other  types  of  valves  and  valve  gear.  Governing  devices  which  vary 
the  cut-off  with  a  single  valve  are  attached  to  the  flywheel  and  their 
operation  depends  upon  the  centrifugal  action  of  heavy  weights  opposed 
to  springs.  As  the  engine  speeds  up  the  centrifugal  force  increases,  over- 
coming the  spring  pressure,  and  the  weights  move  out.  In  doing  so  they 
shift  the  position  of  the  eccentric  on  the  shaft  by  means  of  links. 
The  eccentric  may  be  shifted  as  follows: 

1.  Angle  between  crank  and  eccentric  may  be  changed. 

2.  Eccentricity  may  be  changed. 

3.  Both  the  angle  and  eccentricity  may  be  changed  simultaneously. 
The  first  two  cases  are  almost  never  employed  with  a  single  valve,  but 
it  may  be  well  to  notice  what  the  effect  would  be  if  the  governing  were 
done  in  this  way. 

52.  Changing  Angle  between  Crank  and  Eccentric.     In  Fig.  75  the 
circle  whose  diameter  is  OE  is  the  valve  circle  when  the  eccentric  is  set 
for  latest  cut-off,  the  indicator  card  being  the  same  as  that  shown  in 
Fig.  71.     If  now  the  load  were  to  go  off  so  that  the  engine  started  to 
speed  up  the  governor  would  swing  the  eccentric  around  on  the  shaft, 
to  a  position  represented  by  the  circle  on  the  line  OE\.     The  cut-off 
would  thus  be  shortened  to  a  crank  position  OC\  which,  of  course,  is 
working  in  the  right  direction ;  but,  at  the  same  time,  the  lead  has  been 
made  excessive,  and  release  and  compression  have  been  made  too  early. 
Fig.  76  shows  a  card  illustrating  the  conditions  resulting  from  such  a 
change.     It  is  evident  from  this  that,  while  cut-off  is  shortened  and  the 
area  of  the  card  is  reduced,  the  cross-hatched  area  which  represents 
work  done  on  the  steam  by  the  piston,  also  the  area  under  the  dotted 
curve  which  represents  work  that  might  have  been  obtained  from  the 
steam  with  a  late  release,  are  both  large,  indicating  an  unsatisfactory 
performance. 

53.  Changing  Eccentricity.    A  change  in  cut-off  might  be  accom- 
plished by  reducing  the  eccentricity,  keeping  the  angle  between  crank 
and  eccentric  constant.     Such  a  method  would  be  subject  to  objections 
similar  to  those  for  changing  the  angle  and,  furthermore,  would  not  be 
quite  as  simple  to  accomplish  mechanically. 


GOVERNING  DEVICES   FOR   SINGLE-VALVE   ENGINES 

R, 


67 


FIG.  76. 


68 


MECHANISM   OF  STEAM  ENGINES 


54.  Changing  Both  Eccentricity  and  Angle  between  Crank  and  Eccen- 
tric. Practically  all  flywheel  governors  are  arranged  in  such  a  way 
that  when  the  engine  speed  increases  the  angle  between  the  crank  and 
eccentric  is  changed  and  the  eccentricity  is  changed  at  the  same  time. 
Fig.  77  is  a  line  drawing  of  such  a  governor  of  simple  construction. 
The  eccentric  instead  of  being  attached  directly  to  the  shaft  is  pivoted 


FIG.  77. 


to  the  flywheel  by  the  pin  P.  The  arm  B  is  fast  to  the  eccentric.  By 
means  of  the  pin  A  and  link  L  the  arm  B  is  connected  to  the  governor 
weight  W  which  is  fast  to  the  end  of  the  spring.  In  the  position  shown 
the  center  of  the  eccentric  is  at  E.  As  the  speed  of  the  engine  increases 
W  swings  out  from  the  center,  causing  A  to  swing  up  in  an  arc  of  a 
circle  relative  to  P,  thus  swinging  E  down  in  an  arc  relative  to  P.  The 


GOVERNING  DEVICES   FOR  SINGLE-VALVE   ENGINES  69 

effect  of  this  movement  is,  of  course,  to  increase  the  angle  between  crank 
and  eccentric  and  decrease  the  eccentricity.  ^ 

The  location  of  the  point  of  support  P  relative  to  the  crank  pin  has 
a  very  important  influence  upon  the  way  in  which  the  governor  controls 
the  steam  distribution,  the  effect  on  the  lead  being  particularly  noticeable. 
P  may  be  located  on  a  line  passing  through  the  center  of  the  crank  pin 
and  the  center  of  the  shaft,  as  is  the  case  in  Fig.  77,  or  it  may  be  off 
this  line.  Furthermore,  the  location  of  the  pivot  point  P  determines 
whether  the  lead  shall  increase,  decrease  or  remain  practically  constant 
when  the  cut-off  is  shortened. 

For  purposes  of  discussion  we  will  assume,  for  the  present,  that  the 
point  of  support  is  on  the  line  through  the  center  of  the  crank  pin  and 
shaft  or  that  some  other  means  of  guiding  the  eccentric  is  used  which  is 
equivalent  to  swinging  it  about  a  point  on  this  line.  We  have,  then, 
three  cases  to  consider  as  follows: 

1.  Lead  increasing  as  cut-off  shortens. 

2.  Lead  decreasing  as  cut-off  shortens. 

3.  Lead  remaining  constant  as  cut-off  shortens. 

55.  Increasing  Lead.     Fig.  78  shows  how  the  eccentric  would  be  sup- 
ported relative  to  the  crank  pin  to  produce  increasing  lead  as  the  cut-off 
shortens.     If  the  arc  on  which  the  center  of  the  eccentric  moves  is  con- 
cave toward  the  shaft  the  lead  will  always  increase  as  the  cut-off  shortens. 
In  Fig.  78  it  is  assumed  that  the  valve  takes  steam  on  the  outside  and  is 
driven  direct  from  the  eccentric  rod.     Fig.  79  is  the  Zeuner's  diagram 
for  this  eccentric.     The  full  valve  circle  corresponds  to  the  eccentric 
with  center  at  E  (Fig.  78),  giving  late  cut-off;   this  gives  zero  lead,  cut- 
off at  OC  (Fig.  79),  release  at  OR  and  compression  at  OK.    The  dotted 
valve  circle  is  for  the  eccentric  center  shifted  to  E\  (Fig.  78),  giving  a 
large  lead,  cut-off  at  OCi,  release  at  ORi  (very  early)  and  compression 
at  OKi  (excessive).     The  card  for  this  is  shown  in  Fig.  80,  and  it  is  evi- 
dent that  the  area  representing  lost  work  is  large. 

A  serious  objection  to  a  governor  which  gives  an  increase  of  lead  is 
that  it  will  never  entirely  shut  off  the  steam  supply,  however  far  the 
eccentric  swings  and,  consequently,  cannot  be  depended  upon  to  prevent 
the  engine  "running  away. "  This  type  of  governor  is  rarely,  if  ever, 
used. 

56.  Decreasing  Lead.     Fig.  81   shows  an  eccentric  so  guided  that 
the  lead  decreases  from  an  amount  AB  when  set  for  latest  cut-off  to  zero 
when  swung  down  so  that  it  is  180  degrees  ahead  of  the  crank.     It  is 


MECHANISM  OF  STEAM  ENGINES 


FIG.  78. 


FIG.  79. 


GOVERNING  DEVICES   FOR  SINGLE-VALVE   ENGINES  71 


FIG.  80. 


Connected  to  Governor 


P'LD 


FIG.  Si. 


MECHANISM  OF  STEAM  ENGINES 


FIG.  82. 


FIG.  83. 


GOVERNING  DEVICES  FOR  SINGLE-VALVE   ENGINES  73 

to  be  noted  that  the  arc  over  which  the  eccentric  center  moves  is  now 
convex  toward  the  shaft  and  such  a  condition  will  always  give  decreasing 
lead  with  shortening  cut-off  if  the  point  of  support  is  on  the  center  line 
through  crank  pin  and  shaft. 

Fig.  82  is  the  Zeuner's  diagram  and  Fig.  83  is  the  card  corresponding 
to  the  position  of  the  eccentric  E\.  The  position  of  the  crank  and 
piston  for  the  different  events  is  evident  from  the  diagram  and  from  the 
card.  While  there  is  still  a  considerable  area  on  the  card  representing 
lost  work  the  conditions  are  better  than  in  Fig.  80.  Furthermore  when 
the  eccentric  is  shifted  to  the  limit  the  port  is  never  opened  and  the  engine 
can  get  no  steam.  This  is  an  extremely  important  feature  from  the 
standpoint  of  safety  especially  with  a  condensing  engine.  The  governor 
shown  in  Fig.  77  is  of  this  type. 

57.  Constant  Lead.     If  the  point  of  support  is  kept  on  the  center  line 
of  the  crank  but  is  carried  farther  from  the  center  of  the  shaft  the  arc 
over  which  the  center  of  the  eccentric  moves  becomes  flatter  and  the  lead 
will  be  changed  less  for  a  given  change  of  cut-off.     If  a  method  is  used 
for  guiding  the  eccentric,  which  is  equivalent  to  swinging  it  about  a  point 
on  the  center  line  of  the  crank  at  an  infinite  distance  from  the  shaft, 
then  the  path  over  which  the  center  of  the  eccentric  moves  relative  to 
the  crank  pin,  when  the  cut-off  is  changed,  is  a  straight  line  perpendicu- 
lar to  the  center  line  of  the  crank.     This  means  that  the  lead  will  remain 
unchanged.     Fig.  84  shows  such  an  eccentric  (the  guiding  mechanism 
being  omitted).     Fig.  85  is  the  Zeuner's  diagram  for  the  same.     A  form 
of  governor  which  moves  the  eccentric  in  this  way  will  be  shown  later 
in  connection  with  a  four- valve  engine.     (Fig.  127.) 

58.  Valve  Driven  •  Through  a  Rocker.     The  foregoing  cases  have  all 
assumed  that  the  valve  was  direct  connected  and  that  the  point  of 
support  was  on  the  center  line  of  the  crank  either  at  some  finite  distance 
or  at  infinity.     If  the  path  of  motion  of  the  valve  is  not  parallel  to  that 
of  the  piston  or  if  the  valve  is  driven  through  a  bent  rocker,  such  as 
shown  in  Fig.  10,  then  in  order  to  obtain  the  equivalent  of  the  above 
conditions  a  special  construction  would  have  to  be  made  to  find  the  point 
of  support. 

59.  Point  of  Support  Not  on  the  Center  Line  of  Crank.     It  is  not 
uncommon  to  find  the  point  of  support  at  one  side  of  the  center  line 
of  the  crank  with  direct  connection  to  the  valve.      This  is  shown  in  the 
Zeuner's  diagram,  Fig.  86.     It  is  to  be  noted  here  that  the  full  gear  lead 
is  small  while  the  "mid-gear"  lead  is  zero.     When  the  cut-off  begins  to 


74 


MECHANISM  OF  STEAM  ENGINES 


FIG.  84. 


FIG.  85. 


GOVERNING  DEVICES  FOR  SINGLE-VALVE  ENGINES 


75 


decrease  the  lead  increases  for  a  time,  then  decreases,  so  that  the  valve 
does  not  open  the  port  at  all  when  the  eccentric  is  j8o°  with  the  crank. 
This  diagram  is  for  the  governor  shown  in  Fig.  87.  Another  fact  to  be 


Pivot  is  I  j"  radially 
centre 


FIG.  86. 

noticed  here  is  that  no  eccentric  is  used,  the  eccentric  being  replaced  by 
a  pin  attached  to  the  governor  arm.  The  pin  E,  of  course,  revolves  about 
the  center  of  the  shaft  when  the  wheel  turns  and  drives  the  eccentric 
rod  in  the  same  way  as  would  an  eccentric  whose  center  was  at  E.  This 
is  an  advantage  on  a  high-speed  engine  since  an  eccentric  running  at 
high  speed  is  liable  to  cause  trouble.  The  position  of  the  short  heavy 
arm  which  carries  the  pin  E  is  controlled  by  the  outer  weight  arm  through 
the  short  link.  As  the  speed  increases  the  outer  arm  swings  relative 
to  the  flywheel  and  in  turn  swings  the  eccentric  arm  about  P,  thus  chang- 
ing the  position  of  E  relative  to  the  center  of  the  shaft  and  crank. 
;  60.  Inertia  Governors.  No  governors  which  depend  solely  upon 
centrifugal  force  for  their  operation  can  be  very  sensitive  because  in 
order  to  cause  the  weights  to  move  out  the  speed  must  increase  an 


76 


MECHANISM   OF  STEAM  ENGINES 


FIG.  87.     Governor  for  American-Ball  Engine. 


FIG.  88.     Governor  for  Ridgway  Engine. 


GOVERNING  DEVICES   FOR  SINGLE-VALVE   ENGINES  77 

appreciable  amount.  The  governor  shown  in  Fig.  88  (also  shown  in 
Fig.  1 8)  belongs  to  a  class  known  as  inertia  governors.  In  Fig.  88  the 
centrifugal  force  of  the  weight  arm  (whose  center  of  gravity  is  at  the 
point  C)  just  balances  the  force  of  the  spring  in  any  given  position  when 
the  engine  is  running  at  normal  speed.  This  balance  is  adjusted  by  chang- 
ing the  initial  tension  on  the  spring  and  by  adding  or  taking  away 
weight  from  the  ends  of  the  weight  arm,  provision  being  made  for  doing 
this.  With  the  wheel  turning  as  shown  by  the  arrow,  the  weight  is 
pulled  around  by  the  spring.  Suppose  now  that  the  wheel  is  turning 
at  the  speed  for  which  the  mechanism  is  adjusted.  If  the  load  were  to 
suddenly  go  off  so  that  the  wheel  started  to  speed  up  it  would  try  to  speed 
the  weight  up  with  it.  The  inertia  of  the  weight  tends  to  keep  it  at  its 
former  speed,  the  result  being  that  the  spring  is  stretched  a  little;  that 
is,  the  wheel  goes  a  little  ahead  of  the  weight.  This  is  equivalent  to 
swinging  the  pin  E  in  over  the  dotted  arc,  relative  to  the  center  line  of 
the  crank.  This  pin  E  takes  the  place  of  an  eccentric,  as  was  the  case 
in  the  last  governor  considered.  The  result,  therefore,  of  the  above 
motion  of  E  relative  to  the  center  line  of  the  crank  is  to  increase  the 
angle  between  crank  and  eccentric  and  decrease  the  eccentricity.  D  is 
a  dash-pot  whose  piston  is  connected  to  the  governor  weight  by  the  rod 
K,  its  purpose  being  to  steady  the  action  of  the  weight.  This  type  of 
governor,  on  account  of  its  sensitiveness,  requires  a  well-balanced  valve 
which  is  easily  moved. 


CHAPTER  VI 
RIDING  CUT-OFF  VALVES  AND  THEIR  GOVERNING   DEVICES 

61.  In  the  discussion  of  governing  devices  in  Chapter  V  it  was  pointed 
out  that  the  speed  of  the  engine  is  commonly  controlled,  under  varying 
loads,  by  a  shaft  governor  which  varies  the  cut-off.     It  was  also  apparent 
that  with  a  single  valve,  such  as  the  plain-slide  valve,  which  controls  all 
the  events  of  the  stroke,  any  change  in  the  cut-off  is  accompanied  by  a 
corresponding  change  in  the  other  events.     For  example,  Fig.  90  shows 
a  card  for  a  cut-off  at  |  stroke  resulting  from  a  shifting  of  the  eccentric 
by  a  shaft  governor  with  a  single  valve.     The  release  comes  when  the 
piston  is  at  Mi  and  the  compression  at  N\  instead  of  at  points  near  the 
end  of  the  stroke  as  in  the  full  gear  card,  Fig.  89. 

It  may  be  very  desirable  that  release  and  compression,  having  been 
determined  for  a  certain  engine  running  at  a  given  speed,  shall  remain 
unchanged,  whatever  the  cut-off,  as  represented  by  Fig.  91.  This  can 
only  be  accomplished  by  having  the  valve  which  controls  the  exhaust 
independent  of  the  governor,  and  using  a  separate  valve  or  valves  to 
control  the  cut-off.  Several  different  kinds  of  valves  are  used  for  this 
purpose,  examples  of  which  will  be  taken  up  later.  In  the  present 
chapter  we  shall  discuss  the  type  known  as  riding  cut-off  valves,  often 
spoken  of  as  double  valves. 

62.  Cut-off  Valve   in   Separate   Chest.     The  arrangement  formerly 
used  was  to  provide  a  double  steam  chest  as  indicated  by  Fig.  92.     The 
outer  chest  Ci  was  supplied  with  steam  from  the  boiler.     This  steam  in 
order  to  get  into  the  real  steam  chest  C  was  obliged  to  pass  through  the 
passage  P\.     The  main  valve  V  was  designed  to  give  the  desired  lead, 
release,  and  compression,  and  a  late  cut-off.     The  cut-off  valve  V\  was 
driven  by  a  separate  eccentric  and  was  adjusted  to  close  the  passage  PI 
at  the  time  cut-off  was  desired,  thus  shutting  off  the  steam  supply  even 
though  the  main  valve  had  not  cut  off.     The  eccentric  driving  the  cut- 
off valve  was  controlled  by  a  shaft  governor.     This  arrangement  allowed 
a  very  limited  range  through  which  cut-off  could  be  varied  because  of 

78 


RIDING  CUT-OFF  VALVES  AND  THEIR  GOVERNING  DEVICES        79 


A-*- 


K 


FIG.  89. 


FIG.  go. 


/H- 


FIG.  91. 


8o 


MECHANISM  OF  STEAM  ENGINES 


FIG.  92. 


1 


FIG.  93. 


RIDING  CUT-OFF  VALVES  AND  THEIR  GOVERNING  DEVICES       8 1 

the  fact  that  the  passage  P\  must  be  opened  before  the  admission  by 
main  valve  for  the  other  end  of  the  cylinder. 

63.  Riding  Cut-off  Valve.  Fig.  93  shows  the  details  of  the  valves  and 
Fig.  94  shows  a  diagram  of  the  general  arrangement  of  the  whole  valve 
mechanism  for  a  riding  cut-off  valve.  In  Fig.  93  the  middle  portion  of 
the  main  valve  is  designed  like  any  slide  valve;  then  the  edges  T  are 
added  so  that  all  steam  that  goes  into  the  cylinder  ports  must  pass 
through  the  passages  in  the  main  valve.  The  riding  valve  and  the 
eccentric  which  moves  it  are  so  proportioned  and  adjusted  that  the 
riding  valve  covers  the  valve  port  Ph  at  the  time  head-end  cut-off  is 


Main  valve  d'isplacement=A 

Plate     "  "  =B 

"        "   relative     "    =£ 


FIG.  94. 

desired,  and  covers  the  valve  port  Pc  when  crank-end  cut-off  is  desired. 
The  riding  valve,  having  closed  the  valve  port  for  head-end  cut-off 
must  keep  it  closed  until  the  main  valve  closes  the  head-end  cylinder 
port  else  a  second  admission  of  live  steam  to  the  cylinder  will  occur 
on  the  same  stroke.  The  riding  valve  eccentric  is  controlled  by  a  shaft 
governor  which,  by  shifting  the  eccentric,  causes  the  riding  valve  to 
cover  the  valve  port  at  a  different  time.  Fig.  93  shows  the  riding  valve 
in  its  middle  position  on  the  main  valve.  In  this  position  the  right- 
hand  edge  of  the  riding  valve  is  at  a  distance  Ci,  called  the  clearance,  from 
the  edge  of  the  head-end  valve  port.  The  riding  valve  must,  therefore, 
be  displaced  a  distance  C\  toward  the  head  end  relative  to  the  main  valve 
in  order  for  the  head-end  valve  port  to  be  closed  to  produce  head-end 


82 


MECHANISM  OF   STEAM  ENGINES 


cut-off.  The  valves  are  shown  at  cut-off  in  Fig.  95.  In  Fig.  96  the  riding 
valve  is  just  reopening  the  head-end  valve  port.  This  is  called  re-admis- 
sion by  the  cut-off  valve.  During  the  time  that  the  cut-off  valve  has  kept 


Main  valve  abs. displacement— A 

Plate     "      "  "  -B 

"       "    rel.          "  =C 


FIG.  95- 


Main  valve  abs.  displaceffisnt-A 

Plate     "      "  "          =B 

"       "     rel.          "          =G 


FIG.  96. 

the  valve  port  closed,  the  main  valve  has  closed  the  head-end  cylinder 
port. 

64.  Diagram  for  Riding  Valve.  An  inspection  of  Figs.  94,  95  and  96 
will  make  it  evident  that  the  displacement  of  the  riding  valve  relative  to 
the  main  valve  is  the  thing  which  determines  the  time  of  cut-off.  Accord- 


RIDING   CUT-OFF  VALVES   AND   THEIR   GOVERNING   DEVICES        83 

ingly,  if  a  diagram  is  to  be  used  in  connection  with  the  design  of  a  riding 
cut-off  valve  such  a  diagram  should  show  displacements  of  the  riding 
or  "  plate  "  valve  relative  to  the  main  valve  as  well  as  absolute  displace- 
ments of  the  riding  valve.  Both  the  Reuleaux  and  the  Zeuner's  diagrams 
can  be  constructed  to  fulfil  this  purpose.  The  Zeuner's  diagram,  how- 
ever, is  somewhat  more  convenient  in  this  connection  and  we  will  there- 
fore use  that  one. 

In  Fig.  97  let  OC  be  the  crank,  E  the  center  of  the  eccentric  which 
moves  the  main  valve  and  P  the  center  of  the  eccentric  which  moves 
the  riding  valve.  In  the  position  shown,  if  both  valves  have  harmonic 
motion,  the  main  valve  is  displaced  a  distance  OD  toward  the  crank 


M 


FIG.  97. 


end,  while  the  riding  valve  is  displaced  a  distance  OT  in  the  same  direc- 
tion. The  riding  valve  is  therefore  displaced  toward  the  crank  end 
more  than  the  main  valve  by  an  amount  DT,  that  is,  its  displacement 
relative  to  the  main  valve  is  DT. 

Now  draw  a  line  MN  through  E  parallel  to  XcXh.  Then  EK  is  equal 
to  DT  and  therefore  is  equal  to  the  displacement  of  the  riding  valve 
relative  to  the  main  valve. 

In  Fig.  98  the  same  mechanism  is  shown  with  the  crank  turned  through 
the  angle  0  from  the  head-end  dead  point.  The  main  valve  is  now  dis- 
placed a  distance  ODi  toward  the  crank  end  and  the  riding  valve  a 
distance  OTi  toward  the  head  end;  the  riding  valve  is  therefore  dis- 
placed toward  the  head  end  relative  to  the  main  valve  an  amount  D\T\9 


MECHANISM  OF  STEAM  ENGINES 


equal  to  E\K\.  From  these  two  figures  it  will  appear  that  the  motion 
of  the  riding  valve  relative  to  the  main  valve  is  the  same  as  if  the  main 
valve  did  not  move  and  the  riding  valve  were  driven  by  an  eccentric 
whose  eccentricity  is  EP  and  whose  angle  with  the  crank  is  NEP.  This 

being  the  case  it  is  possible  to  represent 
the  relative  motion  by  a  valve  circle  whose 
diameter  is  EP  and  whose  angle  with  the 
line  of  centers  is  equal  to  NEP.  This  is 
shown  in  Fig.  99  where  angle  XjOPz  is  equal 
to  angle  NEP,  Fig.  97,  and  OP2  =  OP3  = 
EP.  In  order  to  distinguish  these  circles 
from  those  which  would  be  used  to  represent 
the  absolute  displacements  of  the  main 
valve  and  riding  valve  we  will  call  these 
circles  which  represent  the  relative  displacements  the  relative  displace- 
ment circles.  Fig.  100  contains  all  three  sets  of  valve  circles.  It  will  be 
noticed  that  the  diameter  of  the  relative  displacement  circle  (OP2)  is 
one  side  of  a  parallelogram  EOP2P  of  which  the  other  side  is  the  diam- 


FIG.  99. 


Absolute  Displacements  of  the 
Main  Valve  toward  Crank  End 


Absolute  Displacements  of 
the  Riding  Valve  toward 
'Crank  End  


Displacements  of 
Riding  Valve  toward 
Crank  End  Relative 

to  Main  Valve 


FIG.  100. 


eter  of  the  main  valve  circle  and  the  diagonal  is  the  diameter  of  the 
circle  which  shows  absolute  displacements  of  the  riding  valve.  Also 
the  diameter  OP3  is  one  side  of  a  parallelogram  OP3EP.  The  use  of  one 
or  the  other  of  these  parallelograms  is  often  convenient  in  constructing 
the  circle. 


RIDING   CUT-OFF  VALVES   AND  THEIR   GOVERNING  DEVICES       85 

65.  Two  General  Classes  of  Riding  Cut-off  Valves.     Riding  cut-off 
valves  may  be  divided  into  two  general  classes:        • 

1.  Constant  clearance  valves. 

2.  Variable  clearance  valves,  commonly  called  Meyer  valves. 

66.  Constant  Clearance  Valves.     The  most  common  form  of  riding 
cut-off  valve  is  that  shown  in  Fig.  93,  where  the  clearances,  having  been 
determined  in  the  process  of  designing  and  setting,  remain  unchanged. 
The  action  of  this  valve  has  already  been  described.     The  governor 
controlling  it  may  be  of  any  one  of  the  types  discussed  in  Chapter  V. 
The  position  of  the  pivot  point  of  the  eccentric  is  of  importance  in  the 
case  of  the  double  valve  as  well  as  of  the  single  valve,  although  the  im- 
portance may  not  be  so  great,  and  a  type  of  governor  which  would  give 
very  unsatisfactory  steam  distribution  with  a  single  valve  may  be  de- 
signed to  give  good  results  with  a  double  valve.     We  will  consider  two 
positions  of  the  pivot  point  in  order  to  study  more  in  detail  the  opera- 
tion of  the  valve  and  the  method  of  applying  the  Zeuner's  diagram  to 
its  design.     These  positions  are:  First,  the  one  which  gives  constant  ab- 
solute travel  to  the  riding  valve;  second,  the  one  which  gives  constant 
travel  of  the  riding  valve  relative  to  the  main  valve. 

67.  Constant  Absolute  Travel  Riding  Valve.     Fig.  101  shows  a  shaft 
governor  used  to  drive  a  riding  cut-off  valve.     The  eccentric  is  supported 
on  the  shaft  and  turns  relative  to  the  axis  of  the  shaft  when  the  governor 
weights  W  move  out  as  the  result  of  increasing  speed  of  rotation.     In 
other  words,  the  pivot  point  for  this  eccentric  is  at  the  axis  of  the  shaft  so 
that  the  governor  merely  changes  the  angle  which  the  eccentric  makes 
with  the  crank.     The  absolute  travel  of  the  riding  valve  is,  therefore, 
constant  and,  as  will  appear  when  we  study  the  Zeuner's  diagram,  the 
relative  travel  varies. 

Let  us  assume  that  we  have  sufficient  data  to  determine  the  valve 
circles  for  the  main  valve  and  locate  the  crank  positions  for  the  events 
of  the  stroke  controlled  by  the  main  valve.  Also,  assume  that  for  the 
riding  valve  we  know  the  absolute  travel,  the  clearances  and  the  travel 
relative  to  the  main  valve.  We  wish  to  find  the  position  of  the  riding 
valve  eccentric  relative  to  the  main  eccentric  and  the  crank  angle  at 
which  the  riding  valve  gives  cut-off.  Then,  assuming  that  the  absolute 
travel  remains  constant,  find  through  what  angle  the  riding  valve  eccen- 
tric must  be  moved  relative  to  the  main  eccentric  to  give  cut-off  at  a 
crank  angle  of  90°  and  find  the  relative  travel  for  that  setting.  We  will 
work  for  the  head  end  only. 


86 


MECHANISM  OF  STEAM  ENGINES 


In  Fig.  102  OE  is  the  main  valve  circle.  The  main  valve  events  for 
the  head  end  are  lettered  in  the  usual  way.  To  get  the  position  of  the 
relative  displacement  circles  construct  the  parallelogram  OV^EPl  with 


FIG.  101. 

OE  as  a  diagonal  and  the  sides  EP\  and  OP\  equal  respectively  to  one 
half  the  relative  travel  and  one  half  the  absolute  travel  of  the  riding 
valve.  On  OF2  thus  found  draw  the  relative  displacement  circle.  Pro- 
duce OF2  making  OVi  =  OF2  and  OVi  will  be  the  diameter  of  the  other 
relative  displacement  circle.  The  angle  EOP\  is  then  the  angle  between 
the  two  eccentrics.  About  0  draw  a  circle  with  radius  equal  to  the  head- 
end clearance  of  the  riding  valve.  This  intersects  the  relative  displace- 
ment circle  OF2  at  F.  Then  OChi  drawn  through  F  will  be  the  crank 
position  for  head-end  cut-off  by  the  riding  valve  because  when  the  crank 
is  in  that  position  the  riding  valve  is  displaced  toward  the  head  end 


RIDING   CUT-OFF   VALVES  AND    THEIR   GOVERNING  DEVICES        87 

relative  to  the  main  valve  an  amount  equal  to  the  head-end  clearance 
and  is  moving  toward  the  head  end  of  the  main  v#lve.  To  solve  the 
second  part  of  the  problem  draw  the  crank  position  OC^  for  the  changed 


K. 


ReA 


FIG.  102. 

cut-off.  When  the  crank  is  at  OCA2  the  main  valve  is  displaced  a  distance 
OD  toward  the  crank  end  and  if  the  cut-off  by  the  riding  valve  is  to 
occur  at  that  time  the  actual  displacement  of  the  riding  valve  must  be 
OD  minus  the  head-end  clearance.  Therefore  measure  in  DT  equal  to 
the  clearance,  and  OT  will  be  the  absolute  displacement  of  the  riding 
valve.  Now  find  the  diameter  OP%  of  a  circle  which  shall  pass  through 
O  and  T  and  which  shall  be  equal  to  OPi  (since  the  absolute  travel 
is  unchanged).  Then  PiOPz  is  the  angle  through  which  the  eccentric 
must  be  moved  relative  to  the  rest  of  the  mechanism  to  shorten  cut- 
off by  the  riding  valve  from  OChi  to  OChz.  By  constructing  a  paral- 
lelogram with  OE  as  the  diagonal  and  OPz  as  a  side  we  get  OF4  as  the 
diameter  of  one  of  the  relative  displacement  circles.  Therefore  OF4  is 
one  half  the  relative  travel  of  the  riding  valve  for  this  setting. 
.  Another  example  of  the  same  type  of  valve  and  governor  is  shown 
in  Fig.  103.  Here  it  is  assumed  that  the  main  valve  data  is  at  hand, 


88 


MECHANISM  OF  STEAM  ENGINES 

Ch, 


RIDING  CUT-OFF  VALVES  AND  THEIR  GOVERNING  DEVICES       89 

also  the  relative  travel  of  riding  valve  when  the  eccentric  is  set  for  the 
riding  valve  to  cut-off  on  both  ends  at  the  same  time  as  the  main  valve 
cut-offs.  The  head-end  clearance  of  the  riding  valve  is  also  known. 
To  find  the  absolute  travel  of  the  riding  valve,  the  crank-end  clearance, 
the  position  of  the  riding  valve  eccentric  relative  to  the  main  eccentric 
when  set  for  cut-off  as  above  stated;  then,  assuming  that  the  governor 
is  to  shift  the  eccentric  so  that  crank-end  cut-off  by  the  riding  valve 
occurs  at  a  known  crank  angle  earlier  in  the  stroke,  to  find  the  angle 
through  which  the  eccentric  must  be  shifted,  the  relative  travel  for  this 
grade,  and  the  position  of  head-end  cut-off  corresponding  to  the  short- 
ened crank-end  cut-off.  The  stroke  line  is  omitted  from  the  figure 
although  it  was  used  in  getting  the  various  crank  positions. 

The  main  valve  circles  are  first  drawn  and  the  crank  positions  for 
main  valve  events  located  from  the  data.  Since  the  relative  travel  of 
the  plate  on  the  valve  is  known  for  longest  cut-off  the  diameter  of  the 
relative  displacement  circle  for  this  setting  must  be  one  half  this  relative 
travel.  If  the  riding  valve  is  to  give  head-end  cut-off  at  the  crank 
position  OCh  (the  same  as  main  valve  head-end  cut-off)  it  must  be  dis- 
placed toward  the  head  end  relative  to  main  valve,  at  that  time,  an 
amount  equal  to  head-end  clearance.  Therefore,  the  relative  displace- 
ment circle  OV^  must  be  drawn  in  such  a  position  that  its  intercept  on 
OCh  will  be  equal  to  the  head-end  clearance.  This  can  be  done  by 
drawing  about  0  an  arc  with  radius  equal  to  the  clearance  and  finding 
the  center  of  a  circle  of  the  required  diameter  which  will  pass  through  O 
and  the  point  where  the  clearance  arc  cuts  OCh.  In  this  way  the  diam- 
eter OVz  is  found  and  from  that  OV\.  Then  by  constructing  the  paral- 
lelogram EOViPi  and  drawing  its  diagonal  OPi  we  find  the  eccentricity 
of  the  plate  eccentric  and  its  setting  for  latest  cut-off.  The  arc  drawn 
about  O  passing  through  the  intersection  of  the  circle  OV\  with  line  OCC 
will  have  a  radius  equal  to  the  crank-end  clearance.  To  get  the  setting 
of  the  eccentric  for  the  shortened  crank-end  cut-off  and  to  find  the 
relative  travel  for  that  setting,  consideration  had  better  be  given  to  the 
actual  position  of  the  valves  rather  than  to  a  purely  geometric  construc- 
tion. OCci  is  the  crank  position  for  this  shorter  crank-end  cut-off.  At 
that  time  the  main  valve  is  displaced  a  distance  OD  toward  the  head 
end  and  if  cut-off  by  the  plate  is  to  occur  then  it  must  be  displaced  an 
actual  amount  equal  to  OD  minus  the  clearance.  Therefore  measure 
in  from  D  the  clearance,  getting  OT  as  the  absolute  displacement  of  the 
plate.  Now,  since  the  eccentricity  of  the  plate  eccentric  is  constant 


90  MECHANISM   OF  STEAM   ENGINES 

• 

it  is  only  necessary  to  find  the  line  OP4,  which  is  the  diameter  of  a  circle 
equal  to  OPi  and  passing  through  O  and  T.  The  diameter  of  the  other 
circle  is,  of  course,  found  by  producing  P40  to  P3  and  the  relative  dis- 
placement circles  are  then  found  for  this  grade  by  constructing  the 
parallelogram  OEP3V3  with  OP3  as  a  diagonal  and  OEi  as  one  side. 
The  crank  position  for  head-end  cut-off  for  this  setting  of  the  eccentric 
is  found  by  drawing  OChi  through  the  intersection  of  the  head-end  clear- 
ance arc  with  the  relative  displacement  circle  OF4. 

Fig.  104  is  another  solution  of  the  same  problem  in  which  the  position 
of  the  point  P4  is  formed  directly  by  constructing  the  parallelogram 
OF3#iP4  instead  of  getting  P4  by  reference  to  the  actual  valve  displace- 
ments. This  construction  is  a  little  shorter  than  that  used  in  Fig.  103, 
but  should  not  be  employed  unless  the  one  using  it  knows  what  he  is 
doing. 

68.  Constant  Relative  Travel  Riding  Valve.     If  the  shifting  eccentric, 
instead  of  being  swung  about  the  center  of  the  shaft,  as  was  the  case  in 
Figs.  101  to  104,  is  supported  at  a  point  coincident  with  the  center  of 
the  main  eccentric,  the  relative  travel  will  be  constant.     A  diagram  for 
such  a  case  is  shown  in  Fig.  105.     Fig.  1 06  is  a  sketch  of  the  main  eccen- 
tric and  of  the  pin  P  on  the  governor  arm  to  which  the  eccentric  rod  for 
the  riding  valve  is  attached  and  which,  therefore,  takes  the  place  of  an 
eccentric.     This  arm  is  pivoted  at  a  point  on  the  flywheel  which  coin- 
cides in  projection  with  the  center  E  of  the  main  eccentric.     Conse- 
quently as  the  governor  weights  swing  out  the  relative  eccentricity  does 
not  change. 

69.  Double  Piston  Valve.     Fig.  107  is  an  example  of  the  riding  cut- 
off valve  principle  applied  to  a  piston  valve  as  used  on  the  Buckeye 
engine. 

70.  Layout  of  Double  Valve.     In  Fig.  108  is  shown  the  layout  in 
longitudinal  section  of  the  valve  for  which  Fig.  103  is  the  diagram. 

The  main  valve  seat  and  the  middle  part  of  the  main  valve  are  laid 
out  as  already  discussed  in  Chapter  IV  for  any  plain  slide  valve.  The 
width  of  the  opening  Lb  is  made  sufficient  so  that  when  the  valve  is  dis- 
placed its  greatest  amount  toward  the  head  end,  as  indicated  by  the  line 
at  #2,  the  opening  bjz  will  be  enough  to  admit  the  steam  properly.  That 
is,  b%k  should  not  be  less  than  am.  The  distance  be  is  enough  to  prevent 
leakage  past  c  into  the  port  when  the  valve  is  in  extreme  position.  The 
same  statements  apply  to  passage  L^bi  and  the  wall  faci. 

The  position  of  the  upper  ends  of  the  valve  passages  is  more  or  less 


X, 


RIDING   CUT-OFF  VALVES  AND   THEIR   GOVERNING  DEVICES        91 

PI 


':  fl     V 


\  X 


x>     \ 


To  Riding  Valve 


Attached  to  .Governor 


Main  Eccentric 


MECHANISM  OF  STEAM  ENGINES 


FIG.  109. 


RIDING   CUT-OFF  VALVES  AND  THEIR   GOVERNING   DEVICES        93 

arbitrary.  It  is  desirable  to  keep  the  valve  as  short  as  convenient;  on 
the  other  hand,  the  valve  passages  should  not  slant  enough  to  prevent 
free  flow  of  the  steam  into  the  cylinder  ports.  Other  proportions  of  the 
valve  are  determined  by  practical  considerations  of  strength  and  con- 
venience in  construction.  In  the  figure  both  valves  are  drawn  in  actual 


C  hi 


K, 


Wf 


w, 


c2 


cct,c 


c2 


E, 


FIG.  no. 

mid-position,  which  condition,  of  course,  never  exists  when  connected 
up  to  run. 

71.  Meyer  Valve.  A  Meyer  valve  is  shown  in  Fig.  109.  The  riding 
valve  consists  of  two  plates  connected  by  a  right  and  left  screw  and  is 
driven  by  a  fixed  eccentric.  The  screw  comes  out  through  the  head 
end  of  the  valve  chest  and  is  provided  with  a  hand  wheel  so  that  by 
turning  the  screw  the  plates  may  be  set  nearer  together  or  separated, 
thus  changing  the  clearances. 


94 


MECHANISM  OF  STEAM  ENGINES 


Fig.  no  is  a  Zeuner's  diagram  for  a  Meyer  valve  where  the  eccentric 
is  set  so  that  the  line  FiOF2,  which  forms  the  diameters  of  the  relative 
displacement  circles,  falls  behind  the  main  valve  cut-off  lines  OCC  and 
OCh.  In  this  particular  diagram  the  clearances  are  chosen  to  equalize 
cut-off  at  J  stroke.  The  maximum  clearance  on  the  crank  end  is  OW& 
and  on  the  head  end  is  OWhi. 


K 


h2 


R 


FIG.  in. 

Fig.  in  is  a  similar  diagram  where  the  diameters  of  the  relative  dis- 
placement circles  FiOF2  fall  ahead  of  main  valve  cut-off  lines. 

72.  Layout  of  Meyer  Valve.  The  layout  of  the  bottom  part  of  the 
Meyer  valve  is  the  same  as  for  a  constant  clearance  valve.  The  plates 
at  the  top  must  be  long  enough  to  seal  at  the  edges  51  and  5i,  Fig.  109, 
when  set  with  least  clearance  and  when  displaced  the  maximum  amount 
relative  to  main  valve. 


CHAPTER  VII 
MULTIPLE-VALVE  ENGINES 

73.  The  majority   of    large   stationary   engines    are   multiple-valve 
engines,  the  most  common  practise  being  to  use  four  valves,  one  at 
each  end  of  the  cylinder  for  admission  and  cut-off,  and  one  at  each  end 
for  release  and  compression. 

The  multiple  valve  engines  discussed  in  the  present  chapter  are  only 
a  few  of  the  many  kinds  in  use,  but  will  serve  to  illustrate  the  principle 
of  the  valve  mechanism  of  such  engines. 

74.  Corliss  Valve  Mechanism.     One  of  the  first  multiple  valve  engines 
was  that  invented  by  George  H.  Corliss.     The  principle  of  the  Corliss 
valve  gear  has  been,  and  still  is,  widely  used  on  large  engines  of  moder- 
ate speed.     The  details  of  the  gear  as  applied  by  the  various  builders 


FIG.  112. 

differ  to  a  considerable  extent,  and  many  of  the  modifications  have  the 
names  of  the  designers  or  builders  prefixed  to  the  word  Corliss. 

Fig.  1 1 2  is  a  longitudinal  section  of  the  cylinder  of  an  engine  having  a 
Corliss  valve  gear.  There  are  four  valve  chests  extending  crosswise  of 
the  cylinder,  one  at  each  corner.  Each  valve  seat  is  an  arc  of  a  cylindri- 
cal surface,  and  each  valve  is  a  portion  of  a  cylinder  resting  on  this 

95 


MECHANISM   OF  STEAM  ENGINES 


F 


j 


MULTIPLE-VALVE   ENGINES 


97 


surface.  These  cylindrical  valves  oscillate  about  their  respective  axes 
to  open  and  close  the  ports.  The  valve  V  controls  admission  and  cut- 
off for  the  crank  end,  V\  admission  and  cut-off  for  the  head  end,  Vi  re- 
lease and  compression  for  the  crank  end,  and  F3  release  and  compression 
for  the  head  end.  A  and  B  are  the  steam  ports  and  C  and  D  the  exhaust 
ports.  The  engine  is  shown  exhausting  at  the  head  end,  and  approach- 
ing admission  at  the  crank  end,  although  no  attempt  was  made  in  the 
drawing  to  place  the  valves  exactly  in  their  proper  relative  positions. 

Fig.  113  is  a  general  view  of  a  Corliss  engine  built  by  the  Hooven, 
Owens,  Rentschler  Company,  Hamilton,  Ohio.  The  main  working 
parts  of  the  valve  mechanism  can  be  seen  from  this  figure,  and  from 


FIG.  114. 

Fig.  114  which  is  an  enlarged  view  of  the  " releasing  gear"  operating  the 
steam  valves. 

A  casting,  called  the  wrist  plate,  oscillates  on  an  axis  bolted  to  the 
center  of  the  cylinder  casting.  Motion  is  given  to  the  wrist  plate  by 
an  eccentric  on  the  main  shaft  through  an  eccentric  rod,  rocker  or 
" carrier,"  and  reach  rod.  Short  cranks  or  " exhaust  arms"  are  keyed 
to  the  stems  of  the  exhaust  valves  and  are  connected  to  the  wrist  plate 
by  the  "exhaust  links."  The  exhaust  valves  are,  therefore,  in  motion 
all  the  time.  The  proportions  of  the  linkage  are  such,  however,  that  the 
motion  of  the  valves  is  as  slow  as  possible  when  they  are  open  and 


98  MECHANISM   OF   STEAM   ENGINES 

closed,  while  at  the  time  they  are  opening  and  closing  the  motion  is 
rapid.  Motion  is  imparted  to  the  steam  valves  from  the  wrist  plate 
through  the  steam  links.  The  steam  links  are  not  attached  directly  to 
the  steam  valves  as  was  the  case  with  the  exhaust  links,  but  move  the 
valves  through  a  releasing  device  shown  in  Fig.  114.  This  releasing 
mechanism  is  so  constructed  that  the  steam  link  pulls  the  valve  open 
quickly  at  the  proper  time,  and  by  releasing  its  hold  on  the  valve  stem 
allows  the  valve  to  close  suddenly  under  the  action  of  external  forces 
when  cut-off  position  is  reached.  The  time  at  which  the  gear  allows 
the  valve  to  close  is  controlled  by  the  governor.  This  is  seen  at  the 
middle  of  the  engine,  and  consists  of  two  heavy  balls  supported  on  arms 
hinged  at  the  top  of  a  vertical  shaft.  The  shaft  is  rotated  through  bevel 
gears  by  the  shaft  carrying  the  small  pulley  which  shows  directly  below. 
The  pulley  is  driven  by  a  belt  from  the  engine  shaft.  The  arms  carrying 
the  governor  balls  are  connected  by  short  links  to  a  collar  on  the  verti- 
cal shaft,  and  as  the  speed  of  rotation  causes  the  balls  to  swing  out 
this  collar  is  drawn  upward.  The  position  of  the  collar  determines  the 
position  of  a  stop  which  causes  the  releasing  gear  to  let  go  its  hold  on 
the  valve. 

75.  Allis  Releasing  Gear.  Fig.  115  is  a  perspective  drawing  and 
Fig.  116  an  orthographic  projection  of  the  head-end  steam  valve  mecha- 
nism on  an  Allis  engine  built  a  number  of  years  ago,  but  shown  here  be- 
cause it  differs  only  in  minor  details  from  similar  gears  made  today,  and 
lends  itself  well  to  an  explanation  of  the  detail  of  action  of  Corliss  gears 
in  general.  The  parts  in  Fig.  115  are  lettered  the  same  as  the  corre- 
sponding parts  in  Fig.  114,  so  that  if  Fig.  115  is  understood  it  will  be 
easy  to  follow  the  action  of  the  gear  which  is  shown  in  Fig.  114. 

Referring  to  Figs.  115  and  116,  the  rod  M  is  the  steam  link  connecting 
the  end  of  the  steam  bell  crank  B  to  the  wrist  plate.  The  wrist  plate 
oscillates  continuously  through  an  angle  of  about  55°,  driven  by  an 
eccentric  as  explained  for  the  engine  shown  in  Fig.  113.  The  mechanism 
is  in  extreme  position  in  Figs.  115  and  116,  with  the  wrist  plate  about  to 
reverse  its  motion.  The  valve  is  closed.  As  the  top  of  the  wrist  plate 
moves  toward  the  left  the  right  arm  of  the  steam  bell  crank  B  rises, 
and  the  left  arm  swings  down.  Pivoted  on  the  pin  TV,  on  the  left  arm 
of  B,  is  the  claw  A  having  near  its  end  a  hardened  steel  block  T.  The 
steam  arm  E  is  keyed  to  the  shaft  V  which  is  really  the  stem  of  the  valve 
and  which,  when  turned,  causes  the  valve  to  turn.  On  the  back  side  of 
E  is  another  hardened  steel  block  C.  A  spring  Y  is  attached  to  B}  and 


MULTIPLE-VALVE  ENGINES 


99 


FIG.  115. 


100  MECHANISM  OF  STEAM  ENGINES 

rests  against  the  outer  arm  of  A  urging  it  inward.  The  steam  bell 
crank  B  swings  just  far  enough  to  allow  the  block  T  to  go  a  little  way 
past  block  C.  Then  as  B  reverses  its  motion  and  begins  to  move  up- 
ward, the  arm  A,  urged  inward  by  the  spring,  causes  the  block  T  to 
hook  under  C,  and,  therefore,  pulls  C  and  E  upward,  opening  the  valve. 
Loose  around  the  hub  of  B  is  a  ring  having  an  arm  H  connected  by  the 
rod  F  to  the  levers  operated  by  the  governor  collar.  As  the  pin  N 
rises,  the  toe  A\,  which  is  a  part  of  claw  A,  strikes  the  stop  S.  This 
causes  the  claw  to  swing  about  pin  TV  with  the  result  that  the  block  T  is 
unlatched  from  C.  Attached  to  E  is  the  rod  P  running  down  to  a  "dash- 
pot."  When  the  claw  unhooks  from  C,  the  suction  of  the  dash-pot 
combined  with  the  weight  of  the  parts  causes  E  to  drop  suddenly  and 
close  the  valve  producing  cut-off  for  that  end  of  the  cylinder.  If  the 
arm  H  is  moved  a  little  to  the  left  (which  occurs  when  the  governor  balls 
move  out)  the  stop  5  is  moved  down  a  corresponding  amount.  Then  A\ 
strikes  it  sooner,  as  B  swings  up,  thus  giving  an  earlier  cut-off. 

76.  Double-ported  Corliss  Valve.     Corliss  valves  may  be  made  double 
ported.     Fig.  117  shows  a  section  of  the  head  end  of  the  cylinder  of  a 
Hamilton  engine.     Steam  enters  past  the  right-hand  edge  of  the  steam 
valve,  and  also  by  the  passage  P  through  the  valve.     The  exhaust  goes 
out  past  the  right  edge  of  the  exhaust  valve  and  through  the  passage  R. 
It  will  be  noticed  on  each  of  these  valves  that  the  bridge  between  the 
two  ports  furnishes  a  support  for  the  valve  throughout  its  entire  length 
at  all  times,  thus  obviating  any  tendency  to  spring. 

77.  Dash-Pot.     Since  reference  has  been  made  to  a  dash-pot  in  con- 
nection with  the  Corliss  releasing  gears  it  may  be  well  to  notice  here  the 
construction  of  one  form  of  dash-pot.     Fig.  118  is  a  section  through  a 
dash-pot  sometimes  used  on  the  Hamilton  engine.     PI  is  a  piston  hav- 
ing a  large  diameter  at  the  upper  part  and  a  smaller  diameter  below. 
The  pin  R  furnishes  the  connection  between  the  piston  and  the  con- 
necting link  P,  Fig.  114.     When  the  piston  is  drawn  up  a  partial  vacuum 
is  formed  under  the  piston,  tending,  of  course,  to  draw  the  piston  down 
as  soon  as  the  upward  force  is  removed  by  the  action  of  the  releasing 
gear.     A  cushion  of  air  under  the  upper  part  of  the  piston  prevents 
shock  when  the  piston  drops  suddenly.     This  air  escapes  through  a 
small  opening,  but  escapes  so  slowly  that  it  is  compressed  and  in  that 
way  cushions  the  piston. 

78.  Limitation  of  Cut-off  with  Single  Wrist  Plate.     If  a  Corliss  engine 
has  but  one  eccentric,  and  one  wrist  plate  to  drive  both  steam  and  ex- 


MULTIPLE-VALVE  ENGINES 


101 


FIG.  117. 


FIG.  118. 


102 


MECHANISM  OF  STEAM  ENGINES 


haust  valves,  the  eccentric  would  ordinarily  have  to  be  set  90°  or  more 
ahead  of  the  crank  in  order  to  give  a  favorable  action  of  the  exhaust 
valves.  This  necessity  imposes  a  limitation  on  the  time  of  cut-off  by 
the  releasing  mechanism.  This  may  be  seen  from  the  diagram  of  such 
a  gear  in  Fig.  119.  X  is  the  center  of  the  eccentric,  and  is  now  on  the 
center  line  of  the  engine.  The  entire  linkage  is,  therefore,  in  its  extreme 
position,  and  a  further  motion  of  the  eccentric  will  cause  the  steam  bell 
crank  B  to  swing  toward  the  left.  If  the  finger  A\  has  not  come  in  con- 
tact with  the  governor  stop  S  by  the  time  the  linkage  reaches  the  position 

ernor 
Steam  Link 

Reach  Rod 


FIG.  119. 

shown,  it  will  not  strike  5  at  all,  and  the  releasing  mechanism  will  not 
operate  at  all.  As  B  swings  back  it  will  gradually  lower  the  steam  arm 
E,  and  close  the  valve  near  the  end  of  the  stroke.  This  will,  of  course, 
give  cut-off,  but  the  cut-off  will  not  be  under  control  of  the  governor. 
In  other  words,  the  "drop"  cut-off  must  occur  by  the  time  the  center 
of  the  eccentric  is  on  the  engine  center  line.  If  the  angle  between  the 
crank  and  eccentric  needs  to  be  XOK  in  order  to  give  proper  exhaust 
action,  then  the  position  OK  is  the  latest  crank  position  at  which  cut- 
off can  be  obtained  and  still  be  kept  under  control  of  the  governor. 
This  difficulty  is  often  overcome  by  using  two  wrist  plates,  one  for  the 
steam  valves  and  the  other  for  the  exhaust  valves.  Each  wrist  plate 
is  driven  by  its  own  eccentric.  The  steam  eccentric  may  then  be  set 
wherever  desired  without  interfering  with  the  exhaust. 

79.  Rice  &  Sargent  Valve  Gear.  Figs.  120  to  123  show  the  steam 
and  exhaust  gears  of  the  Rice  &  Sargent  engines  built  by  the  Providence 
Engineering  Works.  These  are  Corliss  engines  in  which  no  wrist  plates 
are  used.  The  drawings  and  the  greater  part  of  the  following  descrip- 
tion are  taken  directly  from  the  makers'  catalogue. 


MULTIPLE-VALVE   ENGINES 


103 


The  valves,  Fig.  120,  are  of  the  multiported  Corliss  type.  On  small 
and  medium-sized  cylinders  double-ported  valve^  are  used,  both  for 
steam  and  exhaust,  and  on  the  largest  sizes  triple-ported  valves  are  used, 
insuring  ample  port  opening  with  slight  valve  movement.  The  inlet 
valve  gear  is  shown  in  Figs.  121  and  122.  Fig.  121  shows  the  crank- 
end  steam  gear  in  its  extreme  opening  position.  The  latch  A  on  the 
valve  stem  lever  B  is  engaged  with  the  toe  C  on  the  rocker  D.  The  pin 
E  connects  through  the  intermediate  rockers  and  rods  with  the  steam 
eccentric  on  the  engine  shaft,  and  the  pin  F  connects  to  a  similar  gear 
at  the  head  end  of  the  cylinder.  As  the  rocker  D  moves  to  the  right, 
the  toe  C  engages  the  latch  A,  moving  the  inlet  valve  to  open  it,  and; 


Inlet  Steam  Valve 


Inlet  Steam  Valve  Showing  Opening  Edges 


Inlet  Steam  Valve,   Triple  Ported 
FIG.  120. 

raising  the  dash-pot  plunger  which  is  connected  to  the  pin  P.  Cut-off 
is  accomplished  by  the  toe  C  turning  downward  on  its  pivot  spindle  H 
to  release  the  latch  A .  The  spindle  H  has  a  cam  lever  /  rigidly  attached 
in  the  rear,  which  in  turn  is  carried  between  two  hardened  steel  rolls, 
/,/.  These  rolls  turn  on  pins  in  the  cut-off  lever  K,  which  latter  is  a 
part  of  the  collar,  turning  freely  on  the  valve  stem  journal.  The  arm 
L  above,  forming  part  of  the  same  casting  as  the  cut-off  lever  K,  is 
connected  to  the  governor  by  the  rod  M.  This  rod  is  held  firmly  by 
the  governor,  and  does  not  move  unless  there  is  a  change  in  speed 
of  the  engine.  The  rod  N  connects  to  the  valve  gear  at  the  head  end 


104 


MECHANISM   OF   STEAM    ENGINES 


of  the  cylinder.  The  latch  A  is  released  at  some  point  in  the  opening 
movement  of  the  rocker  D  toward  the  right.  This  is  accomplished 
when  the  rise  O  of  the  cam  lever  passes  between  the  cam  rolls  /,/.  It 
is  obvious  that  the  length  of  the  cut-off  depends  upon  the  position  of 
the  cut-off  lever  K,  as  controlled  by  the  governor.  The  further  to  the 
left  the  lever  K,  the  earlier  the  cut-off. 

Fig.  122  shows  the  rocker  D  at  the  extreme  right  of  its  motion.     Re- 
lease has  taken  place,  and  the  valve  is  about  to  be  closed  by  the  pull  of 


FIG.  i2i. 


TIG.  122. 


the  dash-pot.  The  valve  closes  promptly,  and  the  lever  B  turns  to  the 
position  shown  in  Fig.  121.  The  cut-off  lever  K  is  here  shown  in  the 
position  giving  nearly  the  latest  cut-off  which  is  about  three-quarter 
stroke  of  the  piston.  On  the  return  movement  of  the  rocker  D  the  cam 
rolls  /,/  raise  the  cam  lever  I  and  the  toe  C  to  the  engaging  position. 
At  the  latter  part  of  the  movement  of  the  rocker  D  to  the  left,  as  the 
toe  C  passes  under  the  latch  A,  the  latter  is  raised  by  the  toe  sufficiently 
to  clear  the  same,  and  the  latch  then  drops  by  gravity  in  front  of  the  toe 
to  the  engaging  position. 

The  exhaust  valve  gear  is  shown  by  Fig.  123.  The  link  is  inter- 
posed between  the  valve  rod  bell  crank  and  the  exhaust  lever  to  allow 
the  valve  to  pause  at  the  time  the  pressure  upon  it  is  heaviest,  thus 


MULTIPLE- VALVE   ENGINES 


105 


minimizing  the  friction  loss  and  giving  rapid  motion  at  the  time  of  open- 
ing and  closing  the  ports.  Separate  eccentrics  are  used  for  operating 
the  steam  and  exhaust  valves,  permitting  a  long  range  of  cut-off  and 
satisfactory  adjustment  of  release  and  compression. 


FIG.  123. 

It  is  claimed  that  this  gear  can  be  used  with  satisfactory  results  on 
an  engine  running  as  high  as  two  hundred  or  more  revolutions  per 
minute. 

80.  Fitchburg  Four- valve  Engine.  Fig.  124  shows  the  head-end 
steam  and  exhaust  valves  and  the  valve  gear  for  the  Fitchburg  engine. 


FIG.  124.    Valve  Mechanism  of  the  Fitchburg  Engine. 

The  stems  of  the  steam  valves  are  attached  to  sliders  having  cam  grooves. 
Each  slider  is  moved  by  a  rocker  which  swings  about  a  fixed  axis.  The 
lower  ends  of  these  rockers  are  attached  to  each  other  through  a  link, 


io6 


MECHANISM   OF   STEAM   ENGINES 


and  to  an  eccentric  on  the  engine  shaft  by  rods  and  carriers.  The 
upper  ends  of  the  rockers  carry  rolls  fitting  into  the  grooves  of  their  re- 
spective cams.  The  larger  part  of  each  cam  groove  is  nearly  concentric 


A 


FIG.  125. 

with  the  axis  of  the  rocker  which  drives  it,  the  groove  being  just  enough 
off  to  cause  the  driving  pin  and  roll  to  start  the  cam  at  the  beginning  of 

its  movement  so  that  when  the  roll 
reaches  the  reverse  curve,  which  be- 
gins the  quick  travel  of  the  valve, 
the  latter  is  already  in  motion  in  the 
direction  of  its  travel. 

Fig.  125  is  a  section  through  the 
head  end  valve  and  valve  chest,  and 
Fig.  126  an  outside  view  of  the 
valve.  Steam  is  at  both  ends  of  the 
valve,  and  the  area  exposed  to  pres- 
sure is  less  at  one  end  of  the  valve 

than  at  the  other  by  the  area  of  the  valve  stem.  This  small  unbalanced 
pressure  is  just  enough  to  keep  the  cam  against  its  driving  roll.  It  will 
also  close  the  valve  instantly  in  case  the  valve  rod  or  eccentric  rod  or 
the  driving  latch  should  break  or  be  detached  by  accident. 


\ 


/ 


FlG   I26 


MULTIPLE-VALVE   ENGINES 


107 


These  valves  furnish  a  good  example  of  double-ported  piston  valves. 

The  governor  for  this  engine  is  shown  in  Fig.  127^.  This  is  the  gover- 
nor referred  to  in  §  57,  in  which  the  eccentric  is  moved  straight  across 
the  shaft  as  the  weights  swing  out,  keeping  the  lead  constant,  although 
the  mechanism  is  sometimes  so  arranged  that  the  movement  of  the 


FIG.  127.     Governor  of  the  Fitchburg  Engine. 

center  of  the  eccentric  is  in  a  straight  line,  relative  to  the  center  line  of 
the  crank,  but  not  at  an  angle  of  90°.  This  is  done  to  give  a  decreasing 
lead  as  the  cut-off  shortens. 

The  center  of  the  eccentric  is  at  E,  and  it  is  slotted  so  that  it  does  not 
touch  the  shaft.    At  the  two  points,  D  and  DI,  are  pins  by  means  of 


io8 


MECHANISM   OF   STEAM   ENGINES 


which  the  two  weighted  levers  N  and  A7i  are  pivoted  to  the  flywheel. 
These  levers  take  hold  of  the  eccentric  at  the  pins  5  and  R.  (The  lugs 
to  which  these  pins  are  fastened  are  a  part  of  the  eccentric  casting.) 
The  four-bar  linkage  DSRDi  is,  therefore,  practically  a  Watt  straight- 
line  motion.  The  result  of  swinging  the  levers  about  D  and  D\  is  to 
move  the  center  of  the  eccentric  in  nearly  a  straight  line  relative  to 
the  center  line  of  the  crank.  The  weights  H  and  H\  are  pivoted  to  the 
wheel  at  /  and  /i,  and  are  connected  to  the  levers  DS  and  DiR  by  the 
links  KC  and  KiF.  The  springs  L  and  LI  are  attached  at  one  end  M  and 
MI  to  the  wheel,  and  at  the  other  end  to  the  weights  H  and  HI,  thus 
serving  to  hold  the  weights  in  toward  the  center  and  tend  to  keep  the 
eccentric  set  for  latest  cut-off.  As  the  speed  of  the  wheel  increases  the 
weights  H  and  HI  swing  out,  turning  the  levers  DS  and  DiR  about 
the  pins  D  and  DI  so  as  to  carry  the  eccentric  center  E  nearer  the  cen- 
ter line  of  the  crank. 

The  weights  N  and  Ni  merely  act  as  counterweights. 

81.  Mclntosh  &  Seymour  Gridiron  Valves.  Fig.  128  shows  at  the 
left  a  transverse  section  of  the  cylinder  of  a  Mclntosh  &  Seymour  engine, 


FIG.  128.    Valve  Mechanism  of  the  Mclntosh  &  Seymour  Engine. 

the  section  passing  through  the  valves.  At  the  right  is  a  longitudinal 
section  through  the  crank  end  of  the  cylinder.  The  valves  illustrate  the 
form  of  valve  known  as  gridiron.  Each  exhaust  valve  is  a  series  of 
little  plates  cast  together.  The  valve  seat  contains  a  port  for  each 
section  of  the  valve.  The  principle  here  is  evidently  the  same  as  for  a 
double-ported  valve,  only  it  is  carried  further.  The  steam  valves  are 
also  gridiron  valves,  and  are  provided  with  riding  cut-off  valves,  under 


MULTIPLE-VALVE   ENGINES 


109 


FIG.  129.     Sulzer  Valve  Mechanism. 


HO  MECHANISM  OF  STEAM  ENGINES 

control  of  a  flywheel  governor.  The  valves  are  driven  through  a  system 
of  links,  rock  shafts  and  slides  for  transmitting  the  motion  of  the  eccen- 
trics to  the  valves.  The  links  and  rockers  are  so  arranged  that  the 
valves  move  rapidly  when  opening  and  closing  and  remain  practically 
still  when  closed. 

82.  Sulzer  Valve  Gear.  The  Sulzer  Valve  Mechanism  is  used  largely 
in  Europe,  and  is  well  adapted  for  many  types  of  engines.  It  is  a  four- 
valve  mechanism.  Fig.  129  is  a  section  across  the  cylinder  through  one 
pair  of  valves.  The  valves  differ  from  any  that  we  have  thus  far  con- 
sidered, being  of  the  type  known  as  poppet  valves.  They  are  circular 
in  form,. and  open  by  rising  from  their  seats,  thus  allowing  a  large  open- 
ing for  a  small  movement  and  avoiding  friction  due  to  sliding  on  a  seat. 
In  the  figure  the  exhaust  valve  V\  is  closed  and  the  steam  valve  V  is 
open.  Both  valves  are  urged  shut  by  helical  springs  which  press  against 
collars  on  the  stems.  The  valves  are  operated  by  the  rockers  L  and  LI. 
A  shaft  5  is  driven  from  the  engine  shaft.  On  S  is  an  eccentric  whose 
center  is  at  E.  This  eccentric,  by  means  of  the  eccentric  rod,  swings 
the  link  BA  about  the  pin  B  which  is  also  the  axis  of  the  rocker  L.  As 
the  eccentric  draws  the  pin  A  downward  the  right  end  of  the  rocker  L 
is  pushed  down  by  the  toe  of  the  bent  rocker  /  which  is  pivoted  at  A. 
The  left  end  of  L  is  attached  to  the  valve  spindle  which  is,  therefore, 
raised,  opening  the  valve.  Attached  to  the  pin  C  on  the  rocker  /  is 
the  long  link,  CD,  taking  hold  of  the  rocker  /.  This  rocker  swings 
about  the  pin  Ft  the  position  of  which  is  controlled  by  the  governor 
operating  the  shaft  G.  The  rocker  J  is  moved  about  F  by  the  link  HK 
which  connects  H  with  the  pin  K  on  the  eccentric  strap.  The  effect  of 
this  linkage  KHFDC  is  to  swing  I  about  A  at  the  same  time  that  A  is 
being  drawn  down  by  the  main  eccentric  rod.  When  I  has  turned 
about  A  far  enough  to  allow  the  toe  to  slip  off  the  end  of  L,  the  spring 
forces  the  steam  valve  shut  almost  instantly.  As  the  engine  speed  in- 
creases, the  governor  turns  the  shaft  G  to  lower  the  pin  F.  This  causes 
the  toe  of  7  to  slip  off  L  earlier  and  produces  an  earlier  cut-off. 

The  exhaust  valve  is  opened  by  the  rocker  L\.  This  rocker  is  operated 
by  the  cam  W  on  the  shaft  5  acting  on  the  roller  R  which  is  on  the  end 
of  the  long  rod  taking  hold  of  LI  at  P.  The  upper  end  of  this  rod  is 
guided  by  the  link  XY  swinging  about  the  fixed  center  Y. 


CHAPTER  VIII 
HAND-OPERATED   REVERSING  AND    CONTROLLING   GEARS 

83.  The  valve  mechanisms  previously  discussed  have,  for  the  most 
part,  been  those  used  on  stationary  engines.     Such  engines  always  run 
in  the  same  direction,  and  at  practically  a  constant  speed.     The  govern- 
ing devices  for  controlling  the  steam  supply  as  the  load  varies  are  auto- 
matic so  that  little  attention  is  required  when  running. 

On  marine,  locomotive,  and  other  moving  engines  it  must  be  possible 
to  quickly  change  the  speed  and  direction  of  turning  and  to  adjust  the 
steam  distribution  at  will.  The  same  is  true  of  some  small  stationary 
engines,  such  as  hoisting  engines.  This  control  is  accomplished  by 
hand  or,  at  least,  at  the  will  of  the  operator.  The  same  mechanism 
usually  controls  the  steam  distribution  and  the  direction  of  rotation. 

84.  Link  Mechanisms.     In  Fig.  130,  if  E  is  the  center  of  the  eccentric 
and  the  eccentric  rod  is  directly  connected  to  the  stem  of  a  valve  taking 
steam  on  the  outside,  the  engine  crank  will  turn  in  the  direction  indi- 


Crank 


FIG.  130. 

cated  by  the  arrow.  Should  the  eccentric  be  shifted  to  the  position  EI, 
Fig.  131,  the  direction  of  rotation  would  be  reversed.  If  now,  instead 
of  shifting  the  position  of  the  eccentric,  two  eccentrics  are  provided  as 
shown  in  Fig.  132,  and  provisions  made  for  connecting  either  one  to 
the  valve,  the  direction  of  rotation  will  be  that  shown  by  the  full 
arrow  or  by  the  dotted  arrow  according  as  the  full  eccentric  or  the 
dotted  one  is  connected  to  the  valve. 

in 


112 


MECHANISM  OF  STEAM   ENGINES 


Fig.  133  suggests  a  method  by  which  this  was  sometimes  accomplished 
in  the  early  days.  Each  eccentric  rod  was  provided  with  a  hook  which 
could  be  attached  to  a  pin  on  the  valve  stem.  When  the  eccentric  rod 


Crank 


FIG.  131. 


Crank  Pin 


FIG.  132. 

A  was  hooked  on,  the  direction  of  rotation  was  as  indicated  by  the  arrow. 
To  run  in  the  other  direction  the  linkage  must  be  raised  to  attach  the 


To  Reverse  Lever 


FIG.  133. 

other  hook  to  the  pin;  a  process  which  was,  evidently,  inconvenient. 
The  natural  development  was  to  unite  the  two  hooks  into  one  piece  as 
suggested  by  the  dotted  lines  in  Fig.  134.  The  resulting  mechanism, 


HAND-OPERATED   REVERSING  AND   CONTROLLING   GEARS         113 

shown  in  Figs.  135  and  136,  is  known  as  the  Stephenson  Link  Mecha- 
nism. The  general  operation  of  the  mechanism  is  evident.  In  the 
position  of  the  link  shown  in  Fig.  135,  the  ''forward"  eccentric  is  con- 


, — ^Crank  Pin 


FIG.  134. 

trolling  the  valve.     When  raised  to  the  position  shown  in  Fig.  136  the 
"  backing  "  eccentric  controls  the  valve.     When  set  in  some  intermediate 


FIG.  135.     Stephenson  Link,  Full  Gear  Forward. 

position  the  motion  of  the  valve  is  due  to  the  combined  effect  of  both 
eccentrics,  the  result  being  a  change  in  cut-off  and  other  events  of  the 
stroke  somewhat  similar  to  that  obtained  by  a  shifting  eccentric. 


MECHANISM   OF  STEAM   ENGINES 


FIG.  136.     Stephenson  Link,  Full  Gear  Backing. 


FIG.  137.     Stephenson  Link  with  Rocker,  Full  Gear  Forward. 


HAND-OPERATED   REVERSING  AND   CONTROLLING   GEARS         115 


FIG.  138.    Increasing  Lead. 


FIG.  139.    Decreasing  Lead. 


FIG.  140.    Increasing  Lead. 


n6 


MECHANISM  OF  STEAM  ENGINES 


FIG.  141.    Decreasing  Lead. 


FIG.  142.    Increasing  Lead. 


FIG.  143.     Decreasing  Lead. 


HAND-OPERATED   REVERSING  AND   CONTROLLING  GEARS        117 

85.  Link  with  Rocker.    Fig.  137  shows  a  Stephenson  link  driving  the 
valve  through  a  rocker.     The  essential  difference  rjere  is  that  each  eccen- 
tric is  shifted  180°  from  its  former  position  relative  to  the  crank. 

86.  Increasing  and  Decreasing  Lead.    There  is  a  vital  difference  in 


FIG.  144.     Increasing  Lead. 


FIG.  145.     Decreasing  Lead. 


the  manner  in  which  the  eccentrics  may  be  connected  to  the  link.  This 
difference  manifests  itself  particularly  in  its  effect  on  the  lead.  Figs. 
138  to  145  show  the  change  in  the  lead  as  the  cut-off  is  shortened.  It 
will  be  noticed  that  in  every  case  where  the  lead  increases  as  the  gear  is 
hooked  up  to  shorten  the  cut-off,  the  eccentric  rods  are  uncrossed  or  open 


n8 


MECHANISM   OF  STEAM  ENGINES 


when  the  eccentrics  are  toward  the  link,  as  in  Fig.  146,  and  where  the  lead 
decreases  the  rods  are  crossed  when  the  eccentrics  are  towards  the  link,  as  in 
Fig.  147. 


FIG.  146. 


FIG.  147. 


The  position  of  the  eccentrics  relative  to  the  crank  depends  upon  the 
type  of  valve  (that  is  whether  it  takes  steam  on  the  outside  or  inside) 
and  upon  the  connection,  whether  direct  or  through  a  rocker. 

87.  Radius  of  Slot  in  Link.  It  can  be  shown  that  if  the  radius  of  the 
center  line  of  the  slot  in  the  link  is  equal  to  the  length  of  the  eccentric 
rod  the  lead  will  be  alike  on  both  ends.  In  this  connection,  the  length 
of  the  eccentric  rod  is  understood  to  be  the  actual  length  from  the  center 
of  the  eccentric  to  the  center  of  the  link  pins  D  and  A  plus  or  minus  the 
distance  that  the  link  pin  is  back  or  forward  of  the  center  line  of  the  slot. 


B 


FIG.  148.    Gooch  Link. 

88.  Gooch  Link.  Another  form  of  link  mechanism  similar  in  principle 
to  the  Stephenson,  but  differing  in  detail,  is  the  Gooch  link  shown  in 
Fig.  148.  Here  the  link  is  suspended  from  a  fixed  point  #,  and  the 
block  is  attached  to  the  valve  stem  by  the  rod  E,  known  as  the  radius 
rod.  Instead  of  raising  or  lowering  the  link  to  change  the  cut-off  or  to 


HAND-OPERATED   REVERSING  AND   CONTROLLING  GEARS         119 

reverse,  the  block  is  moved  up  or  down  in  the  slot.  It  is  possible  to 
design  this  link  to  give  constant  lead  by  making  the,  radius  of  the  center 
of  the  slot  equal  to  the  length  of  the  radius  rod. 

89.  Allan  Link.     In  the  link  mechanism  known  as  the  Allan  Link  the 
link  itself  is  straight.     There  is  a  radius  rod  as  in  the  Gooch  Link,  and 
both  the  link  and  the  radius  rod  are  suspended  from  arms  on  the  re- 
verse shaft  in  such  a  way  that  when  the  reverse  shaft  is  turned,  part  of 
the  adjustment  is  given  to  the  link,  and  part  to  the  link  block.     The 
link  being  straight  instead  of  curved  is  cheaper  to  make,  and  for  this 
reason  is  still  used  to  some  extent  on  small  engines. 

90.  Radial  Valve  Gears.     Another  type  of  mechanism  for  reversing 
and  hand  control  of   steam  distribution  includes  such  gears  as   the 
Walschaert,    Hackworth,    Marshall    and    Joy.     These    are    sometimes 
called  radial  valve  gears.     While  these  gears  rarely  have  more  than  one 
actual  eccentric,  and  often  none,  the  valve  motion  is  approximately 
that  which  would  be  given  by  the  combined  action  of  two  eccentrics, 
one  with  90°  angular  advance,  that  is,  directly  opposite  the  crank  or 
coinciding  with  the  crank  according  to  the  type  of  valve  and  connection, 
and  giving  to  the  valve  a  movement  either  side  of  mid-position  equal  to 
the  lap  plus  the  lead.     This  motion  is  combined  with  that  from  another 
eccentric,  or  its  equivalent,  at  right  angles  to  the  crank  which  imparts 
enough  additional  motion  to  the  valve  to  give  proper  port  opening. 
The  connection  between  this  second  eccentric  and  the  valve  is  through 
some  system  of  adjustable  levers  and  links,  by  means  of  which  the  action 
of  the  valve  may  be  changed  to  give  different  grades  of  cut-off  or  to 
reverse  the  direction  of  rotation  of  the  engine.     These  gears,  as  ordinarily 
designed,  give  constant  lead  for  all  grades. 

91.  Walschaert  Valve  Gear.    The  most  important  of  the  so-called 
radial  gears  is  that  invented  by  Egide  Walschaerts  in  Brussels,  and 
patented  in  his  name  in  1844.     This  has  been  the  type  of  valve  mecha- 
nism used  on  locomotives  in  Europe  for  many  years,  but  only  recently 
has  it  come  into  general  use  in  this  country.     At  the  present  time,  it  is 
being  very  generally  applied  to  large  locomotives  here.     Fig.  149  shows 
the  mechanism  applied  to  a  locomotive  having  a  plain  D  valve,  and  Fig. 
150  shows  it  applied  to  a  locomotive  with  an  inside-admission  piston 
valve. 

The  principle  of  operation  of  the  mechanism,  and  the  way  it  may  be 
assumed  to  have  developed  from  the  simple  gear  driving  a  valve  through 
a  rocker  directly  from  a  single  eccentric,  may  be  understood  from  the 


I2O 


MECHANISM   OF  STEAM  ENGINES 


HAND-OPERATED   REVERSING  AND   CONTROLLING  GEARS         12 1 


122  MECHANISM   OF   STEAM   ENGINES 

following  discussion.  In  Fig.  151  let  £  be  the  center  of  a  pin  on 
the  return  crank  CE.  This  return  crank  is  essentially  one  piece  with  the 
crank  proper,  extending  out  to  carry  the  pin  E.  The  center  of  E  is, 
therefore,  equivalent  to  the  center  of  an  eccentric  having  eccentricity 
OE  set  at  an  angle  of  90°  with  the  crank  OC.  TP  is  a  rocker  swinging 
about  the  point  T  which  is  fixed  to  the  frame.  The  rod  PA  connects 
the  pin  P  on  the  rocker  to  the  end  of  the  valve  stem.  The  valve  is  a 
" square"  valve  having  neither  steam  nor  exhaust  laps.  The  mecha- 
nism is,  therefore,  a  simple  eccentric  90°  ahead  of  the  crank  driving  a 
slide  valve  through  a  non-reversing  rocker. 

The  engine,  evidently,  would  not  start  in  this  position,  but  if  turned 
slightly  in  the  direction  of  the  arrow,  the  valve  would  move  to  open 
the  head-end  port  for  admission,  and  the  engine  would  run  in  the  direc- 
tion of  the  arrow,  taking  steam  the  full  stroke.  If  now  we  substitute 
for  the  rocker  TF  the  slotted  link  shown  in  Fig.  152,  still  keeping  its 
pivot  at  r,  put  a  block  on  the  rod  A  P  to  fit  into  the  slot  in  the  link,  and 
provide  means  for  supporting  the  end  5  of  the  rod,  we  can  raise  the 
block  to  any  desired  point  in  the  slot.  As  it  approaches  the  pivot  point 
the  travel  of  the  valve  decreases,  changing  the  steam  distribution.  If  P 
is  carried  above  T,  the  direction  of  motion  of  the  valve  is  reversed,  and 
will  be  right  to  run  the  engine  in  the  reverse  direction.  In  other  words, 
by  making  the  position  of  the  pin  P  adjustable,  we  have  provided  a 
means  for  controlling  and  reversing  the  engine. 

In  order  to  secure  admission,  cut-off,  release  and  compression  at  such 
points  in  the  stroke  as  give  most  advantageous  operation  of  the  engine, 
the  valve  must  be  given  laps.  But  if  it  has  laps  and  is  driven  by  a 
single  eccentric,  the  eccentric  cannot  be  at  90°  with  the  crank,  because 
admission  will  come  too  late.  The  eccentric  might  be  set  more  than  90° 
with  the  crank,  if  the  engine  were  to  run  always  in  the  same  direction, 
but  setting  it  ahead  to  adjust  the  events  for  running  in  one  direction 
would  make  matters  still  worse  when  running  reversed.  Consequently, 
the  main  eccentric  is  kept  at  90°  with  the  crank,  and  another  eccentric 
or  some  equivalent  mechanism  is  introduced  to  cause  the  valve  to  be 
displaced  in  the  proper  direction,  the  lap  plus  the  lead,  when  the  crank 
is  at  the  dead  points.  In  Figs.  149  and  150  the  radius  rod,  instead  of 
being  attached  directly  to  the  valve  stem,  is  attached  to  the  rod  JA, 
known  as  the  combining  lever  or  lap  and  lead  lever.  The  upper  end  of 
the  lap  and  lead  lever  is  attached  to  the  valve  stem,  and  the  lower  end 
is  connected  by  the  union  link  //  to  the  crosshead.  In  the  figure,  the 


HAND-OPERATED   REVERSING   AND    CONTROLLING   GEARS         123 


124  MECHANISM  OF   STEAM  ENGINES 

radius  rod  is  held  by  the  lifting  link  in  such  a  position  that  the  pin  P  is 
opposite  the  trunnion  T  about  which  the  link  L  rocks.  The  motion  of 
L,  therefore,  has  no  effect  on  the  valve,  and  K  is  practically  a  fixed  point. 
If  the  crosshead  moves  back  and  forth,  the  lap  and  lead  lever  rocks 
about  K,  and  causes  the  valve  to  travel  a  total  distance  equal  to  the  sum 
of  the  steam  laps  plus  the  sum  of  the  leads.  This  is  accomplished  by 
so  locating  the  point  K  on  the  lever  that 

KA  _  Sum  of  laps  +  Sum  of  leads 
KJ  Crosshead  Travel 

When  the  gear  is  set  in  this  way,  it  is  in  mid  gear,  and  while  the  engine, 
perhaps,  would  not  start  with  this  setting  it  might  run,  if  already  in 
motion,  in  either  direction.  As  soon  as  the  radius  rod  is  lowered,  the 
rocking  of  the  link  imparts  motion  to  K,  thus  giving  a  corresponding 
motion  to  the  valve  in  addition  to  that  which  it  already  has.  When  the 
block  is  down,  the  engine  will  run  in  the  direction  of  the  arrow,  and  when 
the  block  is  above  the  trunnion,  the  engine  will  run  in  the  reverse  direc- 
tion. In  Fig.  150,  the  valve  is  an  inside-admission  piston  valve  and  its 
motion  must  be  the  reverse  of  that  for  the  valve  in  Fig.  149.  Therefore, 
the  point  K,  where  the  radius  rod  takes  hold  of  the  combining  lever,  is 
above  the  end  of  the  valve  stem,  and  if  the  block  is  still  to  be  in  the  lower 
part  of  the  link  when  running  forward,  the  return  crank  pin  E  must  be 
1 80°  from  the  position  which  it  occupied  in  Fig.  149. 

The  layout  of  a  Walschaert  gear  is  more  or  less  a  matter  of  trial. 
Many  minor  locations  may  be  varied  in  the  design  such  as  the  position 
of  F  or  Tj  and  in  this  way  modifications  in  the  action  of  the  valve 
may  be  accomplished.  The  most  satisfactory  way  to  study  the  design 
of  this  gear,  as  well  as  the  Stephenson  link,  is  by  means  of  a  small  model 
which  may  be  used  on  the  drawing  board,  and  in  which  the  proportions 
may  be  varied  at  will,  and  the  resulting  action  of  the  valve  studied. 

Fig.  153,  taken  from  a  pamphlet  published  by  the  American  Loco- 
motive Company,  shows  the  gear  at  the  different  events  of  the  stroke. 

92.  Hackworth  Valve  Gear.  Fig.  154  shows  the  Hackworth  Valve 
Gear  as  used  by  the  C.  W.  Hunt  Co.  on  hoisting  engines.  E  is  the  pin 
of  a  return  crank,  equivalent  to  an  eccentric  with  eccentricity  OE,  in 
line  with  the  crank,  giving  a  valve  motion  equal  to  the  steam  lap  plus 
the  lead  each  side  of  mid-position.  The  lower  end  of  the  lap  and  lead 
lever  EDP  slides  in  a  straight  slot  in  the  piece  L,  which  may  be  tipped 
at  various  angles  about  the  trunnion  T.  When  the  slot  is  vertical,  the 


HAND-OPERATED  REVERSING  AND   CONTROLLING   GEARS         125 


126 


MECHANISM  OF  STEAM   ENGINES 


linkage  is  in  mid  gear,  and  the  engine  may  be  made  to  run  in  either 
direction  by  inclining  L  either  side  of  the  vertical.  The  valve  shown 
here  is  essentially  a  plain  slide  valve  except  that  it  oscillates  instead  of 
sliding  in  a  straight  line. 


FIG.  154.     Engine  with  Hack  worth  Valve  Gear. 

93.   Marshall  Valve  Gear.     Fig.  155  illustrates  the  Marshall  Gear. 
This  is  similar  to  the  Hackworth  except  that  the  point  A  instead  of  be- 


X                        ^ 

\              (~~- 

-^- 

*\li 

\ 

».       \ 

-- f-> 


FIG.  155.     Marshall  Valve  Gear. 


ing  guided  in  a  straight  slot  is  guided  in  an  arc  of  a  circle  by  the  link  BA . 
The  pin  B  is  carried  by  the  arm  RB,  R  being  the  reverse  shaft.  Grades 
of  cut-off  are  obtained  by  turning  the  reverse  shaft  to  carry  B  nearer  the 


HAND-OPERATED  REVERSING  AND  CONTROLLING  GEARS        127 

vertical  line.     When  B  passes  the  vertical  reversal  is  obtained,  full  gear 
backing  being  with  B  at  B\. 


FIG.  156.     Joy  Valve  Gear. 

94.  Joy  Valve  Gear.  Fig.  156  shows  a  linkage  known  as  the  Joy 
Valve  Gear,  which  has  been  used  to  a  considerable  extent,  and  which 
some  authorities  think  will  come  into  use  again,  although  it  is  not  often 
seen  at  present. 


CHAPTER   IX 
VALVE   SETTING 

95.  However  carefully  a  valve  mechanism  may  be  designed,  or  how- 
ever accurately  it  may  be  constructed,  some  adjustment  must  be  made 
before  the  engine  can  run  satisfactorily.     Also  the  angle  between  crank 
and  eccentric,  which  may  be  plainly  shown  on  valve  diagrams,  must  be 
found  by  trial  on  the  actual  engine.     Nearly  all  valve  gears  are  pro- 
vided with  various  adjustments,  and  the  process  of  adjusting  the  gear 
is  called,  "setting  the  valves."     In  setting  valves,  the  engine  is  usually 
turned  over  by  hand,  the  use  of  crowbars  and  jacks  being  necessary  in 
the  case  of  large  engines,  although  often  the  final  adjustment  is  made 
by  use  of  the  indicator.     Reference  will  be  made  to  this  method  later. 

96.  Finding  the  Dead  Points.     Since  the  lead  and  piston  positions 
for  the  events  of  the  stroke  are  all  referred  to  the  dead  points,  it  is 
necessary  that  we  should  be  able  to  set  the  engine  exactly  on  the  dead 
points.     It  must  be  remembered  that  while  the  piston  is  moving  slowly, 
when  near  the  dead  points,  and,  consequently,  some  error  in  placing  the 
crank  would  give  practically  no  piston  motion,  yet  the  valve  is  moving 
rapidly  at  this  time;  hence  the  necessity  for  being  able  to  determine  both 
dead  points  with  precision. 

Before  the  actual  valve  setting  is  discussed,  a  method  for  determining 
the  dead  points  with  accuracy  will  be  given.  Suppose  it  is  desired  to 
set  the  engine  on  the  head-end  dead  point.  Referring  to  Fig.  157,  place 
the  crank  pin  at  A,  making  a  considerable  angle  with  the  center  line. 
Make  a  mark  on  the  crosshead,  and  a  corresponding  reference  mark  on 
the  crosshead  guide.  Take  some  fixed  reference  point  R  and  mark  the 
point  B  on  the  flywheel  that  is  at  the  reference  point.  A  good  refer- 
ence point  is  a  nail  driven  into  a  board,  and  the  latter  fastened  to  the 
floor,  so  that  the  point  of  the  nail  almost  touches  the  rim  of  the  fly- 
wheel. In  the  diagram,  the  crank-pin  circle  is  taken  to  represent  the 
flywheel.  Now  turn  the  engine  in  the  direction  of  the  arrow  until  the 
crosshead  has  moved  to  the  end  of  its  travel  and  then  back  to  its  former 

128 


VALVE   SETTING 


I29 


position,  as  indicated  by  the  reference  marks.  The  crank  pin  is  at  AI, 
the  point  B  has  moved  on  to  Bi,  and  a  new  point  B  is,  opposite  the  refer- 
ence point. 

A  point  not  to  be  overlooked  is  that  allowance  must  be  made  for  back- 
lash, or  lost  motion,  in  the  parts.     As  the  crank,  when  moved  from  A, 


FIG.  157. 

is  pushing  the  crosshead,  thus  taking  up  lost  motion  in  one  direction, 
so  when  we  find  A\,  we  must  take  up  the  lost  motion  in  the  same  direction, 
that  is,  must  push  the  crosshead.  This  is  done  by  turning  the  crank 
somewhat  beyond  AI  and  then  turning  back  to  this  position. 

Having  now  the  points  B  and  BI  marked  on  the  flywheel,  if  this  arc 
is  bisected  the  point  H  is  obtained.  If  H  is  placed  at  the  reference 
point,  making  allowance  for  lost  motion,  the  engine  is  on  the  head-end 
dead  point.  The  crank-end  dead  point  can  be  found  in  a  similar  manner. 

In  setting  valves,  many  different  cases  arise,  single-valve  engines  with 
fixed  eccentrics,  single  valves  controlled  by  governing  devices,  single 
valves  controlled  by  reversing  devices,  riding  cut-off  valves,  multiple 
valves,  controlled  by  an  almost  infinite  variety  of  mechanisms  and  hav- 
ing almost  as  many  different  kinds  of  adjustments. 

While  it  is  not  possible  to  give  definite  and  concrete  instructions  for 
setting  all  valve  mechanisms,  in  a  book  of  this  character,  yet  the  funda- 
mental principles  underlying  all  valve  setting  will  be  discussed.  In 
some  cases,  more  than  one  method  for  doing  the  same  thing  may  be 
possible,  in  which  event  the  method  involving  the  least  amount  of 
turning  the  engine  by  hand,  is  usually  the  preferable  one. 

97.  To  Set  a  Slide  Valve  for  Equal  Leads.  The  first  case  to  be  con- 
sidered will  be  a  slide  valve  driven  by  a  fixed  eccentric  designed  to  give 
equal  lead  of  a  certain  amount.  The  first  step  is  to  adjust  the  length 


130  MECHANISM  OF   STEAM  ENGINES 

of  the  valve  stem  to  give  equal  leads,  and  the  second  is  to  determine  the 
position  of  the  eccentric,  relative  to  the  crank,  to  give  the  required 
lead. 

Loosen  the  eccentric  on  the  shaft  and  turn,  with  its  attached  valve, 
until  the  greatest  opening  of  the  port  on  the  head  end  occurs  and  measure 
it.  Now  turn  the  eccentric  until  the  greatest  opening  of  the  crank-end 
port  occurs  and  measure  that.  The  port  openings  will  probably  be  un- 
equal, and  must  be  equalized  by  changing  the  length  of  the  valve  stem. 
If  the  head-end  opening  is  the  larger,  the  stem  is  too  short,  if  the  crank- 
end  opening  is  the  larger,  the  stem  is  too  long. 

Next  place  the  engine  on  the  head-end  dead  point,  and  if  the  engine 
has  a  D  valve,  driven  direct,  place  the  eccentric  about  90°  ahead  of  the 
crank,  in  the  direction  the  engine  is  to  run,  and  move  eccentric  slowly 
ahead  until  the  lead  opening  is  the  required  amount,  and  fasten  the 
eccentric.  If  the  work  has  been  carefully  done,  the  crank-end  lead  will 
be  the  same  as  the  head-end,  but  it  is  well  to  try  the  crank-end  lead 
again  to  insure  that  no  mistake  has  been  made.  If  the  engine  has  an 
inside-admission  valve  or  is  driven  through  a  reversing  rocker,  the  only 
difference  from  the  above  case  will  be  in  the  position  of  the  eccentric 
relative  to  the  crank. 

98.  To  Set  a  Slide  Valve  for  Certain  Desired  Cut-offs.     Suppose  it  is 
desired  to  adjust  the  valve  mechanism  to  give  cut-off  at  certain  points 
for  head  and  crank  ends.     Turn  the  engine  until  the  crosshead  has 
moved  from  the  head  end  to  the  position  where  cut-off  is  desired,  loosen 
the  eccentric  and  turn  on  the  shaft  until  cut-off  occurs.     Tighten  the 
eccentric  and  turn  the  engine  until  the  crosshead  has  gone  the  required 
distance  from  the  crank  end;    probably  cut-off  will  not  be  found  to 
come  at  this  point.     Now  move  the  valve  one  half  the  distance  neces- 
sary to  cause  cut-off,  by  changing  the  length  of  the  valve  stem,  and  the 
other  half  of  this  distance  by  changing  the  position  of  the  eccentric  on 
the  shaft. 

A  little  thought  will  show  that  by  changing  both  valve  stem  and 
eccentric,  each  one  half  the  required  amount,  the  head-end  cut-off  will 
remain  unchanged  while  that  on  the  crank  end  is  made  to  come  at  the 
desired  point.  To  make  sure  of  the  work,  it  is  well  to  try  the  cut-off  on 
the  head  end  again,  and  see  that  it  does  come  at  the  desired  point. 

99.  Valve  Setting  with  a  Tram.     Valve  setting  is  often  done  by  use 
of  a  tram  and  tram  marks  on  the  valve  stem.     This  method  is  very 
frequently  used  on  locomotive  valves  to  "true  up"  an  engine  that  has 


VALVE   SETTING  131 

become  a  little  "out  of  square."  It  recommends  itself  on  account  of 
its  simplicity.  After  the  tram  is  made,  and  the  marks  are  on  the  valve 
stem,  the  valve  need  not  be  seen,  and,  consequently,  the  steam-chest 
cover  need  not  be  removed,  in  order  to  set  the  valve. 

Fig.  158  shows  one  form  of  tram,  consisting  of  a  piece  of  bar  steel 
bent  and  pointed,  and  the  ends  usually  hardened. 


FIG.  158. 

The  process  of  getting  the  tram  marks  is  as  follows:  A  center-punch 
mark  is  made  on  some  fixed  part  of  the  engine,  as  the  guide  yoke  or  end 
of  the  steam  chest,  and  the  point  A  of  the  tram  is  placed  in  this  punch 
mark.  The  valve  is  next  moved  until  its  edge  is  at  the  edge  of  the  head- 
end port,  and  a  punch  mark  made  on  the  valve  stem,  into  which  the 
point  B  of  the  tram  will  fall.  It  is  now  known  that  whenever  the  point 
B  is  in  this  punch  mark,  the  edge  of  the  valve  is  at  the  edge  of  the  head- 
end port.  A  similar  punch  mark  is  made  on  the  valve  rod,  when  the 
edge  of  the  valve  is  at  the  edge  of  the  crank-end  port.  After  these 
tram  marks  are  made,  evidently  the  valve  need  not  be  seen,  for  a  refer- 
ence to  them  by  means  of  the  tram  point,  will  show  just  how  far  either 
edge  of  the  valve  is  from  its  port. 

100.  Setting  Single  Valves  Controlled  by  Governing  Devices.  As  in 
the  case  of  a  valve  driven  by  a  fixed  eccentric,  the  length  of  the  valve 
stem  must  be  adjusted  and  the  position  of  the  eccentric  found,  either  to 
give  the  desired  leads  or  cut-offs  at  the  desired  points.  The  position  of 
the  eccentric  may  be  determined  by  turning  the  whole  governor  wheel 
and  eccentric,  on  the  shaft,  until  its  position  is  found,  and  then  fastening 
it  there,  getting,  of  course,  the  setting  for  full  gear  (longest  cut-off). 
Again,  the  position  of  the  governor  wheel  carrying  the  eccentric  may  be 
determined  for  one  engine,  and  then  jigs  used  to  cut  all  key  ways,  and 
to  drill  all  holes,  so  that  all  subsequent  governors  will  be  exact  duplicates 
of  the  first,  and  will  be  keyed  to  the  shaft  with  their  eccentrics  in  the 
same  position  relative  to  the  cranks  as  in  the  first  engine. 

Many  governors  do  not  use  an  eccentric,  but  replace  it  by  a  pin,  off 
center,  the  holes,  of  course,  being  drilled  in  jigs  so  as  to  insure  exact  dupli- 


13 2  MECHANISM   OF   STEAM   ENGINES 

cation.  Often  this  pin,  replacing  the  eccentric,  is  made  eccentric,  so 
that  if  it  is  rotated  a  small  part  of  a  turn  and  clamped  in  that  position, 
the  lead  of  the  engine  is  increased  or  decreased.  The  speed  of  the  en- 
gine is  determined  by  the  tension  of  the  governor  spring,  or  springs, 
which  tension  is  always  adjustable.  It  is  obvious  that  the  engine  must 
be  run  under  steam  to  determine  if  the  spring  tension  is  right  to  give 
the  designed  speed. 

101.  Setting  Single  Valves  Controlled  by  Reversing  Devices.  Valve 
setting  with  reversing  devices  is  entirely  similar  to  setting  valves  driven 
by  a  fixed  eccentric,  except  that  the  setting  must  be  made  for  both  full 
gear  forward  and  backing. 

The  Stephenson  link  motion  as  applied  to  locomotives  of  a  few  years 
ago  had  the  eccentrics  fastened  to  the  axle  by  set  screws,  slotted  eccen- 
tric rods,  and  a  screw  adjustment  for  the  length  of  valve  stem.  It  was 
soon  apparent  that  under  the  hard  service  to  which  a  locomotive  is  sub- 
jected, set  screws  would  slip,  slotted  eccentric  rods  would  slip,  and  nuts 
and  turnbuckles  back  off. 

The  usual  practice  today  is  to  place  a  locomotive  on  rollers  and  find 
the  positions  for  all  eccentrics,  mark  these  positions,  remove  the  eccen- 
trics and  cut  key  ways,  so  that  when  the  eccentrics  are  replaced,  they  can 
be  securely  keyed  in  position.  After  a  few  years'  running,  if  it  is  found 
that,  due  to  wear  or  other  causes,  the  positions  of  the  eccentrics  should 
be  changed,  the  keys  are  driven  out  and  off-set  keys  are  put  in.  Thus 
the  positions  of  the  eccentrics  may  be  changed  slightly  and  still  make 
use  of  the  old  key  ways. 

In  the  case  of  a  D  valve,  the  connection  between  valve  and  valve  stem 
is  such  that  no  adjustment  for  the  length  of  the  stem  is  possible.  With 
a  piston  valve,  the  valve  stern  usually  passes  through  the  valve,  the 
latter  being  held  against  a  shoulder  on  the  stem  at  one  end,  by  a  nut  at 
the  other.  In  this  case,  the  length  of  the  stem  may  be  changed  by 
putting  washers  between  the  valve  and  shoulder. 

If  the  length  of  the  eccentric  rods  is  changed,  it  will  be  equivalent  to 
changing  the  length  of  the  valve  stem.  In  modern  locomotives,  this  is 
accomplished  by  heating  the  rods,  and  drawing  them  out  under  a  ham- 
mer or  shortening  by  upsetting,  as  the  case  may  be.  This  is  not  the 
crude  operation  that  it  might  seem,  for  a  skilled  blacksmith  can  change 
the  length  down  to  a  hundredth  of  an  inch. 

In  cases  where  a  rocker  is  used,  the  position  of  the  rocker  shaft  can 
not  be  changed  after  the  bolt  holes  are  drilled,  except  in  a  vertical  direc- 


VALVE  SETTING  133 

tion.     This  is  accomplished  by  putting  shims  under  the  rocker-shaft 
box,  the  valve  adjustment  thus  being  changed  sornewhat. 

If  properly  designed  and  constructed,  a  valve  driven  by  the  Wal- 
schaert  gear  usually  needs  but  one  adjustment,  namely  the  length  of  the 
valve  stem,  or  what  is  equivalent,  the  length  6f  the  radius  rod.  This 
may  be  accomplished  by  heating  and  drawing  out,  or  upsetting.  The 
pin  eccentric  is  exactly  90°  from  the  crank,  and  this  can  be  made  just 
right  when  the  engine  is  built,  by  keying  the  return  crank  in  the  correct 
position. 

102.  Setting   Riding   Cut-off  Valves.     In  the  case  of  riding  cut-off 
valves,  the  practice  is  to  set  the  main  valve  to  give  equal  leads  of  a 
predetermined  amount,  exactly  as  a  single  valve  would  be  set  for  equal 
leads.     The  riding  valve  is  usually  set  to  give  equal  cut-off  at  some 
point  of  the  stroke,  exactly  like  a  single  valve.     When  the  riding  valve 
is  controlled  by  a  shaft  governor,  as  is  commonly  the  case,  the  cut-off  is 
equalized  at  the  middle  of  the  governor's  range  or  else  at  the  point 
where  it  is  expected  the  engine  will  run  the  major  part  of  the  time. 

103.  Setting  Corliss  Valves.     A  Corliss  gear  with  a  single  wrist-plate, 
may  be  taken  to  illustrate  the  principles  of  valve  setting  for  all  the 
various  types  of  multiple-valve  engines  in  use.    The  method  of  pro- 
cedure is  as  follows: 

Remove  the  bonnets  covering  the  ends  of  the  valves.  Reference 
marks  will  be  found  on  the  ends  of  the  valves  and  seats,  giving  the  posi- 
tions of  the  working  edges  of  the  valves  and  ports. 

Adjust  the  length  of  the  eccentric  rod  so  that  the  rocker  will  swing 
equal  angles  on  each  side  of  the  vertical. 

Next,  adjust  the  length  of  the  reach  rod  so  that  the  wrist-plate  will 
swing  equal  angles  each  side  of  the  vertical. 

On  the  wrist-plate  supporting  stud  a  reference  mark  is  usually  to  be 
found  and  two  others,  indicating  extreme  displacements,  are  on  the 
wrist-plate  hub.  If  a  mark  is  made  on  the  hub,  mid  way  between  the 
extreme  marks,  and  then  this  new  mark  is  brought  to  the  reference 
mark,  the  valve  gear  is  in  mid-position.  Place  the  wrist-plate  in  one 
extreme  position,  with  the  dash-pot  plunger  resting  on  its  seat.  Adjust 
the  length  of  the  dash-pot  rod  so  that  the  claw  is  past  the  hook-block, 
about  ^y.  This  is  called  the  latch-clearance.  Repeat  for  the  other 
extreme  position. 

Place  the  wrist-plate  in  mid-position,  with  the  steam-arm  (E  Fig. 
1 1 6)  hooked  up  and  give  the  steam  valves  the  proper  laps  or  clearances, 


134 


MECHANISM  OF   STEAM   ENGINES 


by  adjusting  the  lengths  of  the  steam  and  exhaust  links.  The  following 
table  of  laps  and  leads  for  various  size  Corliss  engines  is  taken  as  rep- 
resentative of  good  current  practice: 

TABLE  OF  LEADS  AND  LAPS  FOR  DIFFERENT  PISTON  DIAMETERS 


Piston  Diameter, 
Inches 

Lead 

Steam  Lap 

Exhaust  Clearance 

8-14 

A 

A 

A 

14-20 

A 

A 

A 

20-26 

A 

! 

A 

26-32 

A 

A 

A 

32-38 

A 

i 

1 

38-44 

A 

T9* 

A 

Place  the  engine  on  either  dead  point,  the  gear  being  connected  up. 
Turn  the  eccentric  on  the  shaft  until  it  is  about  ninety  degrees  ahead  of 
the  crank,  and  then  turn  it  ahead  slowly  until  the  steam  valve  has  the 
proper  lead.  Fasten  the  eccentric  on  the  shaft,  and  turn  the  engine  to 
the  other  dead  point.  If  the  previous  work  has  been  carefully  done, 
the  lead  on  this  end  should  be  the  same  as  on  the  other.  If  it  is  not,  it 
may  be  made  so  by  changing  the  length  of  the  steam  link;  this,  of  course, 
affects  somewhat  the  steam  lap,  but  as  cut-off  is  not  dependent  on  the 
steam  lap,  this  is  not  a  serious  matter. 

Next,  block  the  governor  about  half-way  up  and  move  the  engine 
ahead  until  it  reaches  the  point  where  cut-off  is  desired  for  normal  lead. 
Adjust  the  length  of  the  proper  governor  reach  rod  until  the  stop  re- 
leases the  valve.  Repeat  for  the  other  end. 

Lastly,  with  the  governor  in  its  lowest  position,  and  with  the  wrist- 
plate  in  one  extreme  position,  adjust  the  safety  stop  so  that  the 
hook-block  will  not  be  caught  when  the  wrist-plate  swings  back.  Repeat 
for  the  other  extreme  position. 

104.  Valve  Setting  with  Steam.  Very  often  valves  are  set  by  running 
the  engine  under  steam  and  taking  indicator  cards.  To  the  experienced 
man,  the  indicator  card  tells  just  what  is  wrong  with  the  valve  adjust- 
ment, and  changes  are  made  until  the  best  possible  card  is  obtained. 
It  is,  of  course,  necessary  to  set  the  eccentric  in  its  approximate  position, 
and  to  roughly  adjust  the  valve  gear  before  the  engine  will  turn  over 
under  steam. 


VALVE   SETTING  135, 

However  carefully  the  valves  have  been  set  without  using  steam,  the 
indicator  should  always  be  applied  and  cards  taken  before  the  valve 
setting  is  considered  complete,  and  the  engine  ready  to  run. 

Figs.  159  and  160  show  a  collection  of  indicator  cards,  where  an  ideal 
card  is  given  and  others,  illustrating  the  effect  upon  the  card,  of  the 
most  common  engine  and  indicator  defects.  These  figures  are  useful  for 
reference. 

In  every  well-regulated  engine  room,  the  indicator  is  frequently  ap- 
plied. Thus  faults  in  the  valve-gear  adjustment,  as  well  as  leaking 
pistons  and  valves  and  other  defects  are  quickly  noted  and  corrected. 


i36 


MECHANISM   OF   STEAM  ENGINES 


Ideal  card 


Too  late  cut-off 


Early  admission,  or  too  much  lead  Too  early  release 


10 


Too  little  compression 


No  compression 


14 


Excessive  back  pressure 


Leaky  valves 


FIG.  159. 


VALVE  SETTING 


Too  early  cut-off 


Late  admission,  or  no  lead. 


Too  late  release 


Too  early  compression 


Choked  admission 


Choked  exhaust 


Inertia  of  indicator 


Sticky  indicator 


FIG.  i 60. 


CHAPTER  X 
STEAM   TURBINES 

105.  The  steam  turbine,  like  the  reciprocating  steam  engine,  is  a 
machine  by  means  of  which  steam  is  enabled  to  do  mechanical  work. 
The  principle  of  its  action  is  suggested  by  the  De  Laval  Trade  Mark, 
Fig.  161.  In  its  simplest  form,  the  turbine  consists  of  a  wheel  mounted 


FIG.  161. 

on  a  shaft  and  enclosed  in  a  steam-tight  casing.  Around  the  circum- 
ference of  the  wheel  are  blades,  buckets,  or  vanes.  Steam,  directed 
against  these  vanes,  drives  the  wheel  around.  The  wheel  being  fast  to 
the  shaft,  turns  the  shaft.  Power  is  taken  from  the  shaft  in  the  same 
way  as  from  the  shaft  of  a  reciprocating  engine. 

The  above  is,  of  course,  a  very  elementary  description,  for  the  actual 
construction  of  a  machine  which  makes  this  operation  practicable  in- 
volves many  refinements  and  complications.  It  is  not  our  purpose  to 
discuss,  to  any  great  extent,  the  theoretical  questions  pertaining  to  the 

138 


STEAM   TURBINES 


139 


turbine,  but  rather  to  consider  their  general  principle  of  operation,  and 
some  examples  for  the  purpose  of  studying  the  mechanism  of  the 
moving  parts. 

106.  Expansion  of  Steam.  In  order  to  understand  the  action  of  the 
steam  in  the  turbine  it  is  necessary  to  notice  one  fact  with  regard  to  the 
properties  of  steam,  without  discussing  at  all  the  reason  for  the  same. 
Let  Fig.  162  represent  a  hole  through  a  thick  piece  of  metal  into  which 


FIG.  162. 

steam,  under  pressure,  is  flowing  as  indicated  by  the  arrows.  The 
corners  of  the  opening  are  rounded  at  the  inlet  side  to  prevent  contraction 
of  the  steam  due  to  sharp  corners.  The  hole  grows  larger  toward  the 
outlet  end,  giving  a  gradual  increase  of  area  of  cross  section.  If  water 
were  flowing  through  such  a  passage  as  this,  the  velocity  of  flow  would 
be  decreased  with  the  increase  of  area  of  section.  With  steam,  however, 
the  enlargement  of  section  allows  the  steam  to  expand,  the  expansion 
resulting  in  a  decrease  in  pressure  and  temperature  with  an  increase  in 
velocity.  In  other  words,  some  of  the  heat  energy  contained  in  the  steam 
is,  in  the  process  of  expansion,  transformed  into  kinetic  energy.  This 
property  of  steam  is  made  use  of  in  driving  the  turbine  wheel. 

107.   Classification  of  Turbines.     Steam  turbines  may  be  classified 
as  follows: 

(a)  Single-stage. 

(b)  Velocity-stage  or  velocity-compounding. 

(c)  Pressure-stage  or  pressure-compounding. 

(d)  Combination  of  velocity-stage  and  pres- 

sure-stage. 

2.  Reaction  turbines. 

3.  Combination  of  impulse  and  reaction  turbines. 


Impulse  turbines: 


140  MECHANISM  OF   STEAM   ENGINES 

In  the  impulse  turbines,  the  expansion  of  the  steam  takes  place  in  the 
passages  which  direct  the  steam  upon  the  moving  vanes  of  the  wheel, 
so  that  the  steam  strikes  the  wheel  with  high  velocity  and  drives  it  by 
the  force  of  the  impact.  In  the  so-called  reaction  turbines,  the  steam 
reaches  the  wheel  at  high  pressure,  expands  during  its  passage  through 
the  vanes,  and  leaves  them  with  high  velocity.  The  force  which  causes 
the  wheel  to  turn  is  partly  the  impulse  due  to  the  velocity  with  which  the 
steam  strikes  the  vanes  and  partly  the  reaction  due  to  the  velocity 
with  which  the  steam  leaves  the  wheel. 

108.  Single-stage  Impulse  Turbines.  Fig.  163  is  an  outside  view 
of  a  single-stage  De  Laval  turbine  coupled  directly  to  a  centrifugal  pump. 
Fig.  164  is  a  plan  section  of  a  similar  machine.  The  names  of  the  various 
parts  are  given  beneath  the  figure.  The  position  of  part  of  the  nozzles 
can  be  seen  by  the  caps  in  Fig.  163.  Fig.  165  shows  a  section  through 
one  of  the  nozzles  in  position  and  suggests  the  shape  of  the  vanes  and  the 
way  the  steam  is  directed  into  them  and  issues  from  them.  Fig.  166 
shows  three  buckets  and  the  way  they  are  fastened  to  the  wheel  in  the 
De  Laval  machine. 

Part  of  the  nozzles  are  provided  with  valves  as  shown  in  Fig.  165  so 
that  they  can  be  shut  off  to  reduce  the  supply  of  steam  delivered  to  the 
wheel. 

Referring  again  to  Fig.  164,  the  nozzles  are  inserted  in  the  wall  of  the 
nozzle  chamber  to  which  the  steam  is  supplied.  The  steam  rushes 
through  the  nozzle,  the  passage  through  which  is  of  increasing  diameter 
and  of  such  proportions  that  the  steam  expands  completely  to  exhaust 
pressure  before  issuing  from  the  nozzle.  In  expanding,  as  explained  in 
§  1 06  it  acquires  very  high  velocity,  often  3000  feet  per  second  or  even 
more.  Striking  the  buckets  as  it  leaves  the  nozzles,  the  steam  forces 
the  wheel  around.  For  the  best  efficiency,  the  linear  speed  of  the  buckets 
should  be  one  half  the  speed  of  the  steam  striking  them.  This  condition 
can  hardly  be  fulfilled  in  practice,  for  the  wheel  would  have  to  be  exces- 
sively large  or  else  the  shaft  would  have  to  turn  at  a  very  high  speed. 
In  some  machines,  where  economy  of  steam  is  not  of  prime  importance, 
the  speed  is  cut  down  to  a  practical  point  at  the  expense  of  efficiency. 
In  most  single-stage  turbines,  however,  the  wheel  shaft  is  allowed  to  run 
fast,  to  give  nearly  the  proper  bucket  speed.  The  wheel  shaft  is  then 
geared  to  the  working  shaft  Y  as  in  Fig.  164,  with  reduction  in  angular 
speed  through  the  gears. 


STEAM  TURBINES 


141 


Tc 

.9 


142 


MECHANISM   OF   STEAM   ENGINES 


o> 

*N     eS     u, 
S   J3     «« 


STEAM   TURBINES 


143 


FIG.  165.    Nozzle  and  Vanes  for  Single-stage  Turbine. 


FIG.  166. 


144 


MECHANISM  OF   STEAM   ENGINES 


109.  Velocity-stage  Impulse  Turbines.  Since  it  is  impossible  to  give 
to  turbine  buckets  the  speed  which  they  should  have  to  use  up  all  of  the 
velocity  which  steam  acquires  when  it  expands  through  a  nozzle  from 
high  pressure  to  atmospheric  pressure,  the  steam  necessarily  leaves  the 
buckets  of  a  single-stage  wheel  with  considerable  velocity,  which  means 


gfccatac 


FIG.  167.     Nozzle  and  Vanes  for  Velocity-stage  Turbine. 

wasted  energy.  To  save  a  part  of  this  energy,  various  methods  have 
been  used  to  direct  the  steam  back  upon  the  same  buckets  again,  or  upon 
another  set  of  buckets  on  the  same  wheel.  A  turbine  in  which  this  is 
done  is  a  velocity-stage  or  velocity-compounding  turbine.  Fig.  167 
shows  the  arrangement  used  in  the  De  Laval  velocity-stage  turbine. 
A  and  C  are  two  rows  of  buckets  on  the  turbine  wheel.  B  is  a  row  of 
stationary  vanes  projecting  in  from  the  wheel  casing  or  other  stationary 
support.  The  manner  in  which  the  steam  is  directed  upon  the  second 


STEAM  TURBINES 


145 


set  of  buckets  is  evident  from  the  figure.  Fig.  168  is  a  picture  of  a 
De  Laval  turbine  having  two  velocity  stages.  The  same  company  builds 
a  wheel  having  three  velocity  stages. 


FIG.  168.     De  Laval  Turbine  with  Two  Velocity  Stages. 

110.  Pressure-stage   or   Pressure-compounding  Turbines.      In  the 

two  preceding  types  of  turbines,  the  steam  was  expanded  completely  to 
exhaust  pressure  in  one  set  of  nozzles.  In  the  pressure-stage  turbines, 
there  are  two  or  more  wheels  on  the  same  shaft  each  in  a  separate  cham- 
ber. The  steam  expands  in  part  only  in  the  first  set  of  nozzles,  thus 
acquiring  less  velocity  and  making  it  possible  for  the  wheel  buckets  to 
run  more  nearly  at  the  proper  speed  relative  to  the  speed  of  the  steam 
jet.  The  first  wheel,  therefore,  runs  in  a  chamber  filled  with  steam 
under  somewhat  reduced  pressure.  In  the  diaphragm  separating  the 
first  wheel  chamber  from  the  second  are  nozzles,  or  passages  equivalent 
to  nozzles,  through  which  the  steam  again  expands  and  is  directed 
against  the  buckets  of  the  second  wheel.  The  process  is  repeated  as 
many  times  as  there  are  stages  in  the  turbine. 

Fig.  169  is  a  section  through  a  Rateau  or  De  Laval  turbine  having 


146 


MECHANISM  OF  STEAM  ENGINES 


STEAM  TURBINES 


FIG.  170. 


FIG.  171.     De  Laval  Diaphragm. 


148 


MECHANISM  OF  STEAM  ENGINES 


STEAM  TURBINES 


149 


twelve  pressure  stages.  Fig.  170  is  a  section  of  three  wheels  and  dia- 
phragms near  the  rim.  Fig.  171  is  a  view  of  one»of  the  diaphragms 
showing  the  openings  which  serve  as  nozzles  for  the  second  and  all 
succeeding  stages. 

111.  Combined  Pressure-stage  and  Velocity-stage  Turbines.  In 
this  class  of  turbines,  of  which  the  Curtis,  built  by  the  General  Electric 
Company,  is  a  familiar  example,  there  are  two  or  more  pressure  stages 
as  described  in  §  no  and  each  of  these  pressure  stages  consists  of  two 
velocity  stages.  Fig.  172  is  a  section  of  a  Curtis  Turbine  having  four 


Diaphragm  Limit 


FIG.  173. 

pressure  stages  as  indicated  at  the  top  of  the  drawing.  Each  one  of  the 
wheels  has  two  rows  of  buckets  with  stationary  guide  vanes  between, 
giving  two  velocity  stages  at  each  wheel.  Fig.  173  shows  the  form  of  the 
buckets  and  nozzles.  The  following,  taken  from  the  General  Electric 
Company's  instruction  book  will  explain  the  flow  of  steam  in  the  Curtis 
turbine : 

"  Steam  is  admitted  into  the  turbine  casing  through  the  first  stage 


MECHANISM   OF  STEAM  ENGINES 


nozzle,  which  is  of  the  expanding  type  and  changes  pressure  energy  into 
velocity  energy.  The  steam  pressure  on  the  entrance  side  of  the  nozzle 
is  equal  to  the  boiler  pressure  less  the  small  drop  due  to  steam  pipe 
friction.  The  steam  pressure  on  the  exit  side  of  the  nozzle  is  that  of  the 
first  stage,  usually  much  less  than  half  the  initial  pressure,  depending  on 
the  total  number  of  pressure  stages,  load,  etc.  The  steam  in  passing 
through  the  nozzle  receives  a  high  velocity  energy,  equal  to  the  l  drop 
of  pressure  '  energy,  and  is  directed  by  the  nozzle  into  the  first  row  of 
buckets  in  the  first  pressure  stage.  The  steam  in  passing  through  the 
moving  buckets  of  each  stage  does  so  at  constant  pressure,  but  with  loss 
of  velocity.  The  direction  of  the  steam  is  reversed  on  leaving  the  first 
row  of  buckets,  but  the  steam  is  redirected  by  a  set  of  intermediate 
buckets.  It  then  enters  the  second  row  of  buckets  on  the  first  stage 
wheel  in  its  original  direction  and  is  exhausted  into  the  first  stage  space. 
The  steam,  being  at  a  lower  pressure  in  the  first  stage  space,  is  greater 
in  volume,  so  that  the  aggregate  area  of  the  nozzles  through  which  it 
passes  to  the  second  stage  is  much  greater  than  in  the  first  stage,  the 
buckets  being  higher  and  the  nozzle  arc  greater.  The  second  and  suc- 
ceeding pressure  stages  are  in  other  respects  similar  to  the  first." 

112.  Reaction  Turbines.    Fig.  174,  taken  from  the  Westinghouse- 
Parsons  Instruction  Book  illustrates  the  principle  of  the  reaction  turbine. 


uucxxcc 

\.  // 


Stationary 


I ))  D  J)/J)  ))  ))  J)  )>< 


lades 

g  Blades 
Hades 
g  Blades 


FIG.  174. 

The  steam,  entering  the  turbine  casing  through  the  main  admission  valve, 
flows  first  through  row  i  of  stationary  blades,  expanding  from  initial 
pressure  P  to  a  pressure  PI.  In  thus  expanding  it  attains  a  velocity, 
the  energy  of  which  is  given  up  on  the  moving  blades,  row  2.  In  the 
passage  of  steam  through  the  blades  of  row  2  the  shape  of  the  blades  is 


STEAM   TURBINES 


152 


MECHANISM  OF  STEAM   ENGINES 


STEAM   TURBINES 


153 


such  that  expansion  again  occurs,  the  pressure  dropping  from  PI  to  PZ. 
This  expansion  again  produces  a  velocity,  but  this  t  time  its  effect  is  to 
react  on  row  2  as  the  steam  issues  from  it.  This  cycle  is  repeated  a 
number  of  times  until  exhaust  pressure  is  reached.  It  is  evident,  then, 
that  the  so-called  reaction  turbine  makes  use  both  of  the  impulse  and  the 
reaction  of  the  steam. 


FIG.  177. 

Fig.  175  is  a  longitudinal  section  of  a  typical  Westinghouse-Parsons 
steam  turbine.  Steam  enters  through  the  strainer  at  S,  passes  through 
the  main  admission  valve,  and  enters  the  turbine  at  A .  After  expanding 
through  the  cylinders  Ri,  R2,  R3,  it  passes  down  the  exhaust  chamber  D 
to  the  condenser.  The  rotating  member,  or  rotor,  consists  of  the  parts 
jRi,  R%,  RZ,  and  PI,  P^  PZ-  The  parts  RL,  R^  RZ,  consist  of  steel  drums 


154  MECHANISM  OF   STEAM   ENGINES 

mounted  upon  a  spindle,  in  which  are  inserted  the  rows  of  buckets  or 
blades.  On  the  opposite  end  of  the  spindle  are  the  balance  pistons 
PI,  P2)  PS,  of  such  diameters  as  to  exactly  balance  the  axial  pressure  on 
the  drums  RI,  R*,  R^  the  different  pressures  at  either  end  of  the  respec- 
tive drum  diameter  being  communicated  to  the  corresponding  piston 
faces  by  means  of  the  passages  E\,  Ezy  £3.  The  rotor  revolves  in  the 
stationary  cylinder  which  has  rows  of  guide  blades  corresponding  to  those 
on  the  rotor  but  set  in  the  reverse  positions  (see  Fig.  174).  There  are 
three  large  changes  in  the  diameter  of  the  working  portions  of  the  rotor 
and  cylinder.  These  are  commonly  referred  to  as  the  high  pressure, 
intermediate  pressure  and  low  pressure  cylinders,  respectively,  starting 
with  the  small  diameter  RI.  Each  cylinder  is  divided  into  small  steps, 
each  one  of  these  steps  having  blade  rows  of  the  same  height.  Each  of 
these  steps  is  known  as  a  barrel,  there  being  usually  three  to  five  barrels 
in  each  cylinder  and  anywhere  from  one  to  twenty  rows  of  blades  in  each 
barrel. 

Fig.  176  is  a  section  of  an  Allis-Chalmers  turbine  of  the  Parsons  type. 
This  is  the  same  in  principle  as  the  Westinghouse-Parsons  but  differs  in 
detail  of  construction.  Fig.  177  shows  the  buckets  and  guide  vanes  for 
the  same  machine. 

113.  Combination  of  Impulse  and  Reaction  Turbines.  Fig.  178  is 
a  section  of  a  "  Double  Flow  "  Westinghouse  Turbine.  Steam  enters 
as  shown,  strikes  first  upon  a  two-velocity-stage  wheel  of  the  same  type 
as  used  in  the  Curtis  machine,  then  flows  each  way  to  a  wheel  of  the 
regular  Parsons  type.  All  three  wheels  are  on  the  same  shaft.  Since 
the  steam  flows  in  both  directions  on  the  Parsons  wheels  it  is  possible  to 
balance  the  end  thrust  without  the  use  of  balance  pistons. 


STEAM  TURBINES 


155 


CHAPTER  XI 
TURBINE  VALVE  MECHANISMS  AND  GOVERNORS 

114.  The  steam  supply  to  a  turbine,  like  that  to  a  reciprocating  engine, 
must  be  under  the  control  of  some  mechanism  which  will  automatically 
adjust  the  supply  to  the  load  on  the  turbine.     There  are  three  principal 
ways  in  which  this  is  accomplished. 

1.  The  steam  enters  the  turbine  through  one  large  valve  and  the 
amount  of  opening  through  this  valve  is  regulated  by  a  centrifugal 
governor  driven  from  the  shaft.     Governors  of  this  type  are,  therefore, 
simply  throttling  governors. 

2.  The  steam  enters  through  a  series  of  small  valves  and  a  centrifugal 
governor  opens  as  many  of  these  valves  as  are  needed  to  furnish  the 
steam  supply. 

3.  The  steam  enters  through  a  large  valve  which  alternately  opens  and 
closes,  the  length  of  time  that  it  is  open  and  the  amount  of  opening 
depending  upon  the  amount  of  steam  needed,  and  being  regulated  by  a 
centrifugal  governor. 

In  addition  to  the  main  governing  mechanism,  there  is  also  some  safety 
device  which  automatically  shuts  off  the  whole  of  the  steam  supply  in 
case  the  main  governor  fails  to  exert  sufficient  control. 

The  throttling  governors  act  much  on  the  same  principle  as  that 
described  for  a  reciprocating  engine,  although  the  details  of  the  mecha- 
nism are  different.  Two  examples  of  the  other  methods  of  governing  will 
serve  to  illustrate  the  general  principle  of  turbine  speed  control. 

115.  Valve  Gear  on  Curtis  Turbine.     Several  different  valve  mecha- 
nisms and  governors  are  used  on  the  Curtis  turbines.     A  description  of 
one  of  these  follows: 

Referring  to  Fig.  172,  steam  is  admitted  to  the  steam  chest,  through 
a  main  throttle  valve  (not  shown  in  the  drawing).  From  the  steam 
chest  the  steam  is  admitted  to  a  series  of  ports,  through  which  it  flows 
to  the  first  stage  nozzles.  Each  of  the  ports  is  covered  by  a  valve. 
If  the  load  on  the  turbine  is  such  that  full  capacity  is  required  all  of 
these  valves  are  open,  but  if  the  load  is  lighter,  the  governor  automat- 

156 


TURBINE  VALVE  MECHANISMS  AND   GOVERNORS 

ically  closes  some  of  the  valves,  thus  depriving  part  of   the  nozzles 
of  steam.  9 

The  mechanism  by  which  the  valves  are  operated  and  the  steam 


FIG.  179. 


supply  controlled  is  shown  in  Figs.  179  and  180.  It  should  be  noticed 
that  the  letters  do  not  correspond  in  the  two  figures.  This  need  not 
lead  to  confusion,  however,  if  in  reading  the  following  discussion,  care  is 


158 


MECHANISM   OF   STEAM   ENGINES 


rfross  Head 
Highest  Positio 

"it-Gr&ss- 


FIG.  i 80. 


TURBINE   VALVE   MECHANISMS   AND    GOVERNORS  159 

taken  to  observe  which  figure  is  being  referred  to  when  any  letter  is 
mentioned. 

Referring  to  Fig.  179,  the  drive  rod  A  is  actuated  by  a  crank  on  a 
shaft  driven  from  the  main  shaft.  This  rocks  the  lever  B.  The  latter 
carries  two  pawls  which  show  plainly  and  which  are  moving  up  and 
down  as  B  oscillates.  Referring  now  to  Fig.  180,  the  pawls  are  lettered 
A  and  D.  K  is  a  crosshead  on  the  end  of  the  stem  of  one  of  the  valves. 
It  is  here  shown  in  its  uppermost  position,  and  has  been  pushed  up  to 
that  position  by  the  pawl  A  on  its  upward  stroke.  The  valve  is,  there- 
fore, open  and  admitting  steam.  As  the  arm  which  carries  the  pawls 
swings  down  the  projection  G  of  the  lower  pawl  D  moves  away  from  the 
piece  E  which  has  evidently  swung  'D  about  its  pivot.  As  soon  as  G  is 
clear  of  E  a  spring  (not  shown  in  Fig.  180,  but  apparent  in  Fig.  179) 
swings  D  back  so  that  its  lower  point  H  strikes  against  the  piece  pro- 
vided for  it  on  the  lower  end  of  the  crosshead  and  pushes  the  crosshead 
down,  closing  the  valve.  With  the  " shield  plate"  E  in  the  position 
shown,  therefore,  the  valve  opens  at  every  up  stroke  of  the  lever  which 
carries  the  pawls,  and  closes  at  every  down  stroke  of  the  same.  The 
position  of  the  shield  plate  E  is  controlled  by  the  governor.  If  the  speed 
of  the  turbine  increases,  the  governor  will  swing  the  shield  plate  up. 
The  projection  G  of  the  opening  pawl  A  will  then  rest  on  E  and  hold  the 
pawl  in  such  a  position  that  the  toe  H  will  not  catch  to  carry  the  cross- 
head  up.  That  is,  when  the  governor  swings  E  up  the  closing  pawl  D 
is  free  to  close  the  valve,  but  the  opening  pawl  A  cannot  open  it.  Now, 
assume  that  with  the  steam  shut  off  from  the  nozzles  controlled  by  the 
valve,  the  machine  slows  down.  The  governor  will  move  the  shield 
plate  down  past  the  neutral  position  until  the  valve  opens.  If,  when 
a  valve  opens,  the  steam  admitted  is  still  insufficient  to  maintain  speed, 
the  shield  plates  continue  to  move  downward  until  the  shield  plate 
controlling  the  next  valve  to  the  right,  which  is  higher  in  position, 
permits  this  valve  to  open.  The  same  principle  holds  true,  of  course, 
with  the  machine  speeding  up,  if  when  a  given  valve  closes  there  is  still 
more  steam  than  is  necessary  to  maintain  speed  with  the  given  load. 
It  is  understood  that  the  pawls  oscillate  independently  of  the  changes  of 
position  of  the  shield  plate  and  usually  with  much  greater  frequency. 

Fig.  179  shows  one  valve  element  with  its  operating  mechanism.  On 
a  complete  valve  gear  of  the  500  kw.  3600  r.p.m.  machine  there  are  six 
or  eight  complete  elements  (depending  on  steam  conditions),  and  the 
successive  operation  of  these  valves  is  provided  for  by  staggering  the 


160  MECHANISM   OF  STEAM   ENGINES 

shield  plates,  the  one  to  the  extreme  left  (facing  as  the  steam  flows  into 
the  turbine)  is  the  lowest,  each  successive  shield  plate  being  about  J 
inch  higher  than  its  neighbor  to  the  left.  The  left-hand  valve  is,  there- 
fore, the  no-load  valve. 

It  can  be  seen  that  the  motion  imparted  by  the  pawl  to  the  crosshead 
is  transmitted  through  the  nut  T  (on  opening)  and  the  compression 
spring  V  (on  closing)  to  the  adjusting  nut  S  and  thence  to  the  valve 
stem  /. 

The  governor  is  contained  in  the  casing  which  shows  at  the  left  in 
Fig.  179.  The  rocker  EF  and  the  rod  G  form  the  linkage  by  means  of 
which  the  governor  controls  the  position  of  the  shield  plates.  Fig.  181 
contains  two  views  of  the  governor.  The  upper  view  is  a  plan  view 
showing  the  governor  in  its  casing,  with  the  lid  cut  away  to  show  the 
working  parts.  The  lower  view  is  not  a  straight  transverse  section, 
but  two  sections.  The  part  to  the  left  of  the  center  line  is  taken 
through  one  of  the  pivots,  to  the  center;  the  part  to  the  right  of  the 
center  line  is  a  section  through  the  stud  /  for  one  of  the  links  L,  the 
transmission  link  seat  F,  U,  Pa  to  the  center.  The  spindle  at  the  center 
is  driven  from  the  main  shaft  by  a  worm  and  wheel.  As  the  spindle 
speed  increases,  the  governor  weights  swing  out  spreading  the  links  H 
and  Qa,  thus  drawing  down  the  left  end  of  the  rocker  which  adjusts  the 
shield  plates. 

The  spring  (upper)  is  shown  at  N  and  one  of  its  plugs  at  0,  the  bolt 
holding  the  plug  on  which  the  knife  edge  R  is  mounted  is  shown  at  P. 
These  bolts  can  be  screwed  in  and  out  of  the  plug  for  the  purpose  of 
obtaining  different  tension  adjustments  on  the  springs  and  are  checked 
with  lock  nuts.  The  knife  edge  seat  T  is  bolted  to  the  weight  B  (lower 
figure)  and  the  weights  are  caused  to  move  in  unison  by  two  links  L 
connecting  the  studs  /  through  ball  bearings  M.  The  manner  of  sup- 
porting the  transmission  links  and  the  details  of  the  transmission  are 
clearly  shown  and  require  no  description  except  that  the  cage  Y  revolves 
and  that  the  ball  Ba  is  stationary  with  reference  to  rotation  and  has 
vertical  motion  only. 

116.  Valve  Gear  on  Westinghouse-Parsons  Turbine.  Steam  is  ad- 
mitted to  the  Westinghouse-Parsons  turbine,  Fig.  175,  through  the 
primary  valve  V\  under  ordinary  load  and  also  through  the  secondary 
valve  Vz  in  case  of  overload.  The  governor  is  seen  at  the  extreme 
right.  Fig.  182  is  a  section  through  the  primary  and  secondary  valves. 
When  the  throttle  valve  on  the  main  steam  pipe  is  open,  steam  fills  the 


TURBINE  VALVE  MECHANISMS  AND   GOVERNORS  161 


FIG.  181. 


162  MECHANISM   OF   STEAM   ENGINES 

spaces  Vi  and  Vz.  We  will  first  give  attention  to  the  primary  valve.  In 
order  that  the  steam  may  go  from  V\  into  the  main  inlet  to  the  turbine, 
the  primary  poppet  valve  must  be  raised  from  its  seat.  This  is  accom- 
plished by  steam  flowing  through  the  little  passage  A  and  forcing  the 
piston  C  upward,  carrying  the  valve  stem  and  valve  with  it  and  thus 
admitting  steam  to  the  turbine.  At  the  right  is  a  relay  plunger  F  which 
is  moving  up  and  down  constantly,  like  the  slide  valve  of  a  reciprocating 
engine.  This  relay  plunger  is  moved  from  a  cam  or  eccentric  on  a  shaft 
driven  from  the  main  shaft.  The  connection  is  through  a  rockshaft  and 
system  of  levers  so  arranged  and  so  connected  to  the  governor  that  the 
latter  controls  the  position  of  the  stroke  of  the  relay  valve  F.  When 
the  turbine  is  running  at  normal  speed  and  under  normal  load,  the  relay 
valve  is  in  such  a  position  that  on  its  down  stroke  it  opens  the  outlet  of  the 
passage  E,  allowing  the  steam  under  the  piston  C  to  exhaust  through  the 
relay  steam  chest.  The  compression  spring  H  in  the  dash-pot  above 
the  piston  C  then  forces  the  poppet  valve  shut,  stopping  the  flow  of 
steam  to  the  turbine.  On  the  up  stroke  of  F  the  passage  E  is  closed, 
steam  pressure  accumulates  under  C  and  the  poppet  valve  again  opens. 
The  poppet  valve  is,  therefore,  traveling  up  and  down,  admitting  steam 
to  the  turbine  in  puffs.  If  the  load  becomes  lighter,  the  mean  position 
of  the  relay  plunger  is  lowered  by  the  governor,  allowing  a  greater  ex- 
haust of  steam  through  E  and  allowing  the  spring  H  to  close  the  poppet 
valve  correspondingly.  With  a  heavier  load,  the  mean  position  of  the 
relay  plunger  is  raised,  giving  less  exhaust  through  E  and  a  correspond- 
ingly greater  and  longer  opening  of  the  poppet  valve.  With  light  loads 
no  more  pressure  accumulates  beneath  the  piston  C  than  is  sufficient  to 
just  raise  the  valve  from  its  seat  at  each  down  stroke  of  the  plunger. 
As  the  load  increases,  the  valve  has  an  increasing  lift  until  at  maximum 
load  the  puffs  of  steam  merge  into  a  continuous  blast,  and  the  valve 
remains  practically  stationary  in  its  wide-open  position.  In  case  of 
overload,  when  the  primary  valve  is  not  able  to  supply  sufficient  steam, 
the  secondary  valve  opens,  admitting  steam  directly  to  the  second 
cylinder.  The  general  mechanism  of  the  secondary  valve  is  similar  to 
that  of  the  primary  valve,  and  its  relay  valve  is  operated  by  levers  on  the 
same  rockshaft  that  operates  the  primary  valve.  The  secondary  valve 
is  likely  to  be  inoperative  for  long  periods  at  a  time,  however,  since  it 
only  operates  in  case  of  heavy  load.  For  this  reason,  the  relay  plunger 
is  so  constructed  that  it  does  not  begin  to  exhaust  steam  until  the  action 
of  the  secondary  valve  is  needed.  Steam  is  admitted  to  both  sides  of 


TURBINE  VALVE   MECHANISMS   AND    GOVERNORS 


163 


164 


MECHANISM   OF   STEAM   ENGINES 


the  piston  N  through  the  passage  A2  and  when,  on  account  of  increased 
load,  the  mean  position  of  the  relay  plunger  P  is  raised  sufficiently  by  the 
governor,  the  port  O  is  opened,  exhausting  the  steam  from  the  upper  side 
of  the  piston  N,  and  the  valve  is  raised  by  the  pressure  beneath  N. 


Gov.  Case. 
Gov:  Ball  Lever. 

Gov.  Case  Stand. 
Gov.  Rocker  Arm 

T- 

Gov.  Clutch 
Lever  Link 

Gov.  Rock  Shaft 

Bracket 
Gov.  Starting 
Trigger 


Gov.  Spindle 


Gov.  Bevel  Pinion 


Gov.  Bevel  Gear- 


Gov.  Spring 


'ov.  Knife  Edge 
Pin  and  Block 

Gov.  'Arm 

Gov.  Roller  and  Pin 
Gov.  Connecting  Link 
Gov.  Dash  Pot  Lever 
Gov.  Dash  Pot  Clutch 

Gov.  Clutch 
'Gov.  Vibrator  Rod  Lmlt 
'Gov.  Clutch  Lever 


Gov.  Vibrator  Rod 

-Gov.  Gear  Case 

Vibrator  Roller 

iClL,  Gov.  Vibrator  Cam 

'.  Worm  Wheel  Shaft 

vil  Pump  Crank 
'Gov.  Worm  Wheel 

'.  Worm 


FIG.  183. 


The  port  O  may  be  permanently  closed  by  the  hand  valve  Q,  thus 
cutting  the  secondary  valve  entirely  out  of  action. 

Fig.  183  shows  the  governor  mechanism  as  far  as  the  shaft  T  which 
carries  the  motion  to  the  relay  plungers  through  suitable  levers  and  links. 
The  governor  clutch  lever  F  has  its  fulcrum  on  a  collar  or  "  governor 
clutch  "  which  can  slide  on  the  governor  spindle.  F  is  rocked  about  its 
fulcrum  by  the  governor  vibrator  cam,  rod,  and  link,  and  the  other  end 
of  F  rocks  the  shaft  T  by  means  of  the  governor  rocker  arm  and  the  short 


TURBINE  VALVE  MECHANISMS  AND   GOVERNORS  165 

link.  The  governor  spindle  D  is  driven  from  the  main  shaft  of  the  rotor 
by  the  governor  worm,  worm  wheel  A  and  a  pair  of  bevel  gears.  As  the 
speed  increases,  the  weights  W  swing  out,  and  raise  the  collar  which 
forms  the  fulcrum  of  F.  The  arrangement  of  the  levers  between  T  and 
the  relay  plunger  is  such  that  as  the  fulcrum  of  F  is  raised  the  relay 
plunger  is  lowered.  Since  F  rocks  approximately  the  same  amount  about 
its  fulcrum  for  all  positions  of  the  governor,  the  shifting  of  the  fulcrum  of 
F  merely  changes  the  mean  position  of  the  relay  plunger  without  materi- 
ally changing  the  length  of  its  stroke.  This  change  of  mean  position 
varies  the  steam  supply  as  already  explained. 


IND  EX 

PAGE 

Action  of  reciprocating  engines 2 

Admission 3 

Allan  link up 

Allen  locomotive  valve 36 

Allis-Chalmers  turbine 150 

Allis  releasing  gear 98 

American-Ball  engine, 

valve  for 34 

governor  for 76 

Angle  between  crank  and  eccentric 27, 38 

Angle-compound  engine 17 

Angular  advance 27 

Auxiliary  valve  circle  (see  Relative  displacement  circle) 84 

Balanced  valves 31  to  34 

Ball  engine  (see  American-Ball  engine) 34,  76 

Bell-crank  lever 12 

Bilgram  diagram 45 

Bridges 2 

Buckeye  engine,  valve  for 90 

Bushing  for  piston  valve 32 

Calculations  for  size  of  ports 58 

Carrier 12 

Classification  of  reciprocating  engines  (see  Types  of  engines) 16 

Classification  of  turbines  (see  Types  of  turbines) 139 

Clearance  -<-  exhaust 25 

Clearance  —  riding  valve 81 

Combined  reaction  and  impulse  turbines 154 

Combined  velocity-stage  and  pressure-stage  turbines  (see  Curtis  turbine) 149 

Compound  engines 17 

Compression , . . . : 3 

Condensing  engines 22 

Connecting  rod  and  crank 4 

Constant-absolute-travel  riding  valve 85 

Constant-clearance  riding  valve 85 

Constant  relative-travel  riding  valve 90 

Corliss  valve  mechanism 95 

Crank  positions  for  different  events  of  stroke 4,  27  to  30 

Crank  positions  —  notation  for '$ 

Crosby  indicator 13,  14 

Cross-compound  engine 17 

167 


1 68  INDEX 

PAGE 

Crosshead  displacement 4 

Crosshead  velocity 6 

Curtis  turbine 149 

Curtis  turbine  valve  gear 156  to  161 

Cut-off 3 

Cut-off  valve  in  separate  chest 78 

Dash-pot 100 

D  valve 25,28 

Dead  points 2,  126 

De  Laval  turbines 140  to  147 

Diagrams, 

indicator 12 

for  single  slide  valves 39  to  47 

for  riding  valves 82  to  94 

Displacement, 

of  crosshead 4 

of  valve 10 

Double-flow  turbines 154 

Double-ported  Corliss  valve 100 

Double-ported  slide  valve  (see  Ported  valves) 35 

Double  piston  valve 90 

Double  valves  (see  also  Riding  cut-off  valves) 78 

Eccentric  rod  and  eccentric 9 

Eccentricity ^ 

Ellipse  —  valve 39 

Equal  events 30 

Events  of  the  stroke $, 

Events  of  stroke  —  position  of  mechanism  for 27  to  30 

Expansion  of  steam 3,  139 

Flywheel  governors 65  to  77,  86,  107 

Fitchburg  four- valve  engine  gear 105 

Fitchburg  valve 106 

Fitchburg  governor 107 

Four- valve  engines  (see  Multiple- valve  engines) 95  to  no 

Gooch  link 118 

Governors  for  reciprocating  engines 63  to  77,  86,  107 

Governors  for  steam  turbines 156  to  165 

Gridiron  valves 108 

Hackworth  valve  gear 124 

Hamilton-Corliss  engine  (see  Hooven,  Owens,  Rentschler) 7 

Hand-operated  reversing  and  controlling  gears in  to  127 

Harmonic  motion 7 

Hooven,  Owens,  Rentschler  releasing  gear 97 

Horizontal  engine 17 

Impulse  turbines 139  to  I5° 

Indicated  horse  power 16 

Indicator  cards  (in  Indicator  diagrams) 12,  15,  136,  137 

Indicator  diagrams  (see  Indicator  cards) 12,  15,  136,  137 

Indicator  —  steam  engine I3>  I4 


INDEX  169 

PAGE 

Inertia  governors 75 

Joy  valve  gear 127 

Laps 25 

Layout  of  D  valve 58 

Layout  of  double  valve 90 

Layout  of  Meyer  valve 94 

Lead 25 

Lead  angle 25 

Link  mechanisms 1 1 1  to  1 19 

Mclntosh  &  Seymour  valve  gear 108 

Marshall  valve  gear 126 

Mean  effective  pressure 16 

Meyer  valve 93 

Mid-position 10 

Modifications  of  slide  valve 31 

Multiple-expansion  engines 17 

Multiple-valve  engines 95  to  no 

Non-condensing  engines 22 

Notation  for  crank  positions 3 

Parsons  turbine 150  to  155 

Piston  valve 31,  90 

Plain  slide  valve 25 

Poppet  valve no 

Ports 2 

Port  calculations 58 

Ported  valves 35 

Pressure-compounding  turbines  (see  Pressure-stage  turbines) 145 

Pressure-stage  turbines 145 

Problems  on  slide-valve  engine 48  to  62 

Radial  valve  gears 119  to  127 

Reaction  turbines 139,  150  to  155 

Reciprocating  engine, 

definition  of viii 

description  of i 

Relative  displacement 84 

Relative-displacement  circle 84 

Release 3 

Reuleaux  diagram 44 

Rice  &  Sargent  valve  gear 102 

Ridgway  engine, 

governor  for 76 

valve  for 37 

Riding  cut-off  valve 81  to  94 

Rockers 10  to  12,  73,  117 

Rotary  slide  valve 126 

Setting  valves 129  to  133 

Short  cut-off 57 

Simple  engine i}  17 

Single-stage  turbines 140 


170  INDEX 

PAGE 

Skinner  balanced  valve < 33 

Steam  —  expansion  of 3,  139 

Stephenson  link 113  to  118 

Sulzer  valve  gear 109,  no 

Tandem  compound  engine 7 

Throttling  governor 65 

Tram 130 

Travel  of  valve 9 

Triple-expansion  engines 22 

Turbine  speed  control  —  methods  of 156 

Turbines  —  steam 138  to  155 

Turbines  —  governors  and  valve  gears  for 156  to  165 

Types  of  engines 16 

Types  of  turbines 139 

Valve  displacements 10 

Valve  ellipse 39 

Valve  setting 129  to  133 

Velocity  of  crosshead 6 

Velocity-compounding  turbines  (see  Velocity-stage  turbines) 144 

Velocity-stage  turbines 144 

Vertical  engines 17 

Walschaert  valve  gear 119  to  125 

Westinghouse-Parsons  turbine 150  to  154 

Westinghouse-Parsons  turbine  valve  gear 162  to  165 

Zeuner's  diagram 41  to  44 


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